font-lock: highlight operators with "builtin" face

What face is the most appropriate is arguable. Emacs 29.1 has
"operator-face", but it's unavailable for older Emacs, and even for
newer versions it's just "nil", so no highlight. Kinda pointless face.

The current "variable-name" is definitely wrong though, because
operators are far from being variables. And there's a separate
"color-identifiers-mode" which uses this face to highlight similar
variables, and that would work wrong for purescript-mode.

Thus, replace the highlight to "builtin".

Author: Hi-Angel

purescript-font-lock.el

@@ -128,7 +128,7 @@
 ;; This is probably just wrong, but it used to use
 ;; `font-lock-function-name-face' with a result that was not consistent with
 ;; other major modes, so I just exchanged with `purescript-definition-face'.
-(defvar purescript-operator-face 'font-lock-variable-name-face)
+(defvar purescript-operator-face 'font-lock-builtin-face)
 (defvar purescript-default-face nil)
 (defvar purescript-literate-comment-face 'font-lock-doc-face
   "Face with which to fontify literate comments.

tests/purescript-font-lock-tests.el

@@ -55,13 +55,13 @@ import Data.Either (Either(..))
     (53 58 font-lock-keyword-face)
     (60 70 font-lock-type-face)
     (73 78 font-lock-type-face)
-    (80 81 font-lock-variable-name-face))))
+    (80 81 font-lock-builtin-face))))
 
 (ert-deftest string ()
   (purescript-test-ranges
    "foo = \"hello\""
    '((1 3 font-lock-function-name-face)
-     (5 5 font-lock-variable-name-face)
+     (5 5 font-lock-builtin-face)
      (7 13 font-lock-string-face))))
 
 (ert-deftest multiline-string ()
@@ -71,7 +71,7 @@ hello
 \"\"\"
 "
    '((1 3 font-lock-function-name-face)
-     (5 5 font-lock-variable-name-face)
+     (5 5 font-lock-builtin-face)
      (7 19 font-lock-string-face))))
 
 (ert-deftest multiline-string-with-hash ()
@@ -85,7 +85,7 @@ hello
 \"\"\"
 "
    '((1 3 font-lock-function-name-face)
-     (5 5 font-lock-variable-name-face)
+     (5 5 font-lock-builtin-face)
      (7 114 font-lock-string-face))))
 
 (ert-deftest multiline-string-with-embedded-strings ()
@@ -96,7 +96,7 @@ this = \"still a string\"
 \"\"\"
 "
    '((1 3 font-lock-function-name-face)
-     (5 5 font-lock-variable-name-face)
+     (5 5 font-lock-builtin-face)
      (7 37 font-lock-string-face))))
 
 (ert-deftest docs-bar-comment-different-spacings ()
@@ -163,11 +163,11 @@ noncomment
      (5 5 nil)
      (6 14 font-lock-type-face)
      (15 21 nil)
-     (22 22 font-lock-variable-name-face)
+     (22 22 font-lock-builtin-face)
      (23 23 nil)
      (24 29 font-lock-type-face)
      (30 37 nil)
-     (38 39 font-lock-variable-name-face)
+     (38 39 font-lock-builtin-face)
      (40 40 nil)
      (41 43 font-lock-type-face)
      (44 45 nil))))
@@ -205,47 +205,47 @@ mkMyComponent = do
 "
    '((1 13 font-lock-function-name-face)
      (14 14 nil)
-     (15 16 font-lock-variable-name-face)
+     (15 16 font-lock-builtin-face)
      (17 17 nil)
      (18 26 font-lock-type-face)
      (27 30 nil)
      (31 43 font-lock-function-name-face)
      (44 44 nil)
-     (45 45 font-lock-variable-name-face)
+     (45 45 font-lock-builtin-face)
      (46 46 nil)
      (47 48 font-lock-keyword-face)
      (49 61 nil)
-     (62 63 font-lock-variable-name-face)
+     (62 63 font-lock-builtin-face)
      (64 65 nil)
      (66 72 font-lock-type-face)
      (73 73 nil)
-     (74 75 font-lock-variable-name-face)
+     (74 75 font-lock-builtin-face)
      (76 76 nil)
      (77 79 font-lock-type-face)
      (80 81 nil)
-     (82 83 font-lock-variable-name-face)
+     (82 83 font-lock-builtin-face)
      (84 104 nil)
      (105 119 font-lock-string-face)
      (120 120 nil)
-     (121 121 font-lock-variable-name-face)
+     (121 121 font-lock-builtin-face)
      (122 122 font-lock-keyword-face)
      (123 123 nil)
-     (124 125 font-lock-variable-name-face)
+     (124 125 font-lock-builtin-face)
      (126 126 nil)
      (127 131 font-lock-type-face)
-     (132 132 font-lock-variable-name-face)
+     (132 132 font-lock-builtin-face)
      (133 134 font-lock-keyword-face)
      (135 149 nil)
-     (150 151 font-lock-variable-name-face)
+     (150 151 font-lock-builtin-face)
      (152 152 nil)
      (153 159 font-lock-type-face)
      (160 160 nil)
-     (161 162 font-lock-variable-name-face)
+     (161 162 font-lock-builtin-face)
      (163 181 nil)
-     (182 182 font-lock-variable-name-face)
+     (182 182 font-lock-builtin-face)
      (183 183 nil)
      (184 184 font-lock-type-face)
-     (185 185 font-lock-variable-name-face)
+     (185 185 font-lock-builtin-face)
      (186 192 nil)
      (193 194 font-lock-type-face)
      (195 195 nil))))
@@ -264,43 +264,43 @@ instance semigroupNonEmptyList :: Semigroup (NonEmptyList a) where
 derive newtype instance foldableNonEmptyList :: Foldable NonEmptyList
 "
    '((1 8 font-lock-keyword-face) (9 28 nil)
-     (29 30 font-lock-variable-name-face) (31 31 nil)
+     (29 30 font-lock-builtin-face) (31 31 nil)
      (32 37 font-lock-type-face) (38 38 nil)
      (39 50 font-lock-type-face) (51 51 nil)
      (52 56 font-lock-keyword-face) (57 69 nil)
-     (70 70 font-lock-variable-name-face) (71 71 nil)
+     (70 70 font-lock-builtin-face) (71 71 nil)
      (72 83 font-lock-type-face) (84 85 nil)
      (86 86 font-lock-keyword-face) (87 87 nil)
      (88 89 font-lock-type-face) (90 95 nil)
-     (96 96 font-lock-variable-name-face) (97 101 nil)
+     (96 96 font-lock-builtin-face) (97 101 nil)
      (102 113 font-lock-type-face) (114 119 nil)
      (120 121 font-lock-type-face) (122 137 nil)
      (138 138 font-lock-type-face) (139 139 nil)
      (140 142 font-lock-type-face) (143 147 nil)
      (148 148 font-lock-type-face) (149 149 nil)
      (150 152 font-lock-type-face) (153 158 nil)
-     (159 159 font-lock-variable-name-face) (160 168 nil)
+     (159 159 font-lock-builtin-face) (160 168 nil)
      (169 173 font-lock-keyword-face) (174 196 nil)
-     (197 197 font-lock-variable-name-face) (198 203 nil)
+     (197 197 font-lock-builtin-face) (198 203 nil)
      (204 204 font-lock-type-face) (205 208 nil)
      (209 220 font-lock-type-face) (221 224 nil)
      (225 226 font-lock-type-face) (227 233 nil)
      (234 234 font-lock-type-face) (235 243 nil)
      (244 244 font-lock-type-face) (245 247 nil)
      (248 248 font-lock-type-face) (249 255 nil)
      (256 263 font-lock-keyword-face) (264 286 nil)
-     (287 288 font-lock-variable-name-face) (289 289 nil)
+     (287 288 font-lock-builtin-face) (289 289 nil)
      (290 298 font-lock-type-face) (299 300 nil)
      (301 312 font-lock-type-face) (313 316 nil)
      (317 321 font-lock-keyword-face) (322 332 nil)
      (333 344 font-lock-type-face) (345 348 nil)
      (349 350 font-lock-type-face) (351 360 nil)
-     (361 361 font-lock-variable-name-face) (362 366 nil)
+     (361 361 font-lock-builtin-face) (362 366 nil)
      (367 378 font-lock-type-face) (379 382 nil)
      (383 384 font-lock-type-face) (385 388 nil)
-     (389 390 font-lock-variable-name-face) (391 403 nil)
+     (389 390 font-lock-builtin-face) (391 403 nil)
      (404 409 font-lock-keyword-face) (410 448 nil)
-     (449 450 font-lock-variable-name-face) (451 451 nil)
+     (449 450 font-lock-builtin-face) (451 451 nil)
      (452 459 font-lock-type-face) (460 460 nil)
      (461 472 font-lock-type-face) (473 473 nil))))
 
@@ -319,7 +319,7 @@ foreign importinvalid
      (8 8 nil)
      (9 14 font-lock-keyword-face)
      (15 21 nil)
-     (22 23 font-lock-variable-name-face)
+     (22 23 font-lock-builtin-face)
      (24 24 nil)
      (25 30 font-lock-type-face)
      (31 31 nil)
@@ -329,7 +329,7 @@ foreign importinvalid
      (43 43 nil)
      (44 49 font-lock-keyword-face)
      (50 58 nil)
-     (59 60 font-lock-variable-name-face)
+     (59 60 font-lock-builtin-face)
      (61 61 nil)
      (62 67 font-lock-type-face)
      (68 68 nil)
@@ -339,7 +339,7 @@ foreign importinvalid
      (80 80 nil)
      (81 86 font-lock-keyword-face)
      (87 95 nil)
-     (96 97 font-lock-variable-name-face)
+     (96 97 font-lock-builtin-face)
      (98 98 nil)
      (99 104 font-lock-type-face)
      (105 105 nil)
@@ -398,113 +398,113 @@ arr = 1 : [2,3]
      (24 25 nil)
      (26 30 font-lock-type-face)
      (31 31 nil)
-     (32 33 font-lock-variable-name-face)
+     (32 33 font-lock-builtin-face)
      (34 37 nil)
      (38 40 font-lock-comment-delimiter-face)
      (41 50 font-lock-comment-face)
      (51 52 font-lock-function-name-face)
      (53 53 nil)
-     (54 55 font-lock-variable-name-face)
+     (54 55 font-lock-builtin-face)
      (56 58 nil)
-     (59 60 font-lock-variable-name-face)
+     (59 60 font-lock-builtin-face)
      (61 63 nil)
      (64 65 font-lock-function-name-face)
      (66 66 nil)
-     (67 67 font-lock-variable-name-face)
+     (67 67 font-lock-builtin-face)
      (68 68 nil)
-     (69 69 font-lock-variable-name-face)
+     (69 69 font-lock-builtin-face)
      (70 71 nil)
-     (72 73 font-lock-variable-name-face)
+     (72 73 font-lock-builtin-face)
      (74 77 nil)
      (78 80 font-lock-comment-delimiter-face)
      (81 83 font-lock-comment-face)
      (84 87 font-lock-function-name-face)
      (88 88 nil)
-     (89 89 font-lock-variable-name-face)
+     (89 89 font-lock-builtin-face)
      (90 90 nil)
      (91 92 font-lock-keyword-face)
      (93 97 nil)
-     (98 99 font-lock-variable-name-face)
+     (98 99 font-lock-builtin-face)
      (100 118 nil)
      (119 121 font-lock-comment-delimiter-face)
      (122 130 font-lock-comment-face)
      (131 137 font-lock-function-name-face)
      (138 138 nil)
-     (139 140 font-lock-variable-name-face)
+     (139 140 font-lock-builtin-face)
      (141 141 nil)
      (142 145 font-lock-type-face)
      (146 148 nil)
-     (149 150 font-lock-variable-name-face)
+     (149 150 font-lock-builtin-face)
      (151 153 nil)
-     (154 155 font-lock-variable-name-face)
+     (154 155 font-lock-builtin-face)
      (156 156 nil)
      (157 162 font-lock-type-face)
      (163 163 nil)
      (164 170 font-lock-function-name-face)
      (171 171 nil)
-     (172 172 font-lock-variable-name-face)
+     (172 172 font-lock-builtin-face)
      (173 179 nil)
      (180 182 font-lock-comment-delimiter-face)
      (183 184 font-lock-comment-face)
      (185 193 font-lock-function-name-face)
      (194 195 nil)
-     (196 196 font-lock-variable-name-face)
+     (196 196 font-lock-builtin-face)
      (197 197 nil)
      (198 201 font-lock-type-face)
      (202 202 nil)
      (203 203 font-lock-keyword-face)
      (204 205 nil)
-     (206 206 font-lock-variable-name-face)
+     (206 206 font-lock-builtin-face)
      (207 210 nil)
      (211 213 font-lock-comment-delimiter-face)
      (214 221 font-lock-comment-face)
      (222 225 font-lock-function-name-face)
      (226 228 nil)
-     (229 229 font-lock-variable-name-face)
+     (229 229 font-lock-builtin-face)
      (230 232 nil)
-     (233 233 font-lock-variable-name-face)
+     (233 233 font-lock-builtin-face)
      (234 236 nil)
-     (237 237 font-lock-variable-name-face)
+     (237 237 font-lock-builtin-face)
      (238 238 nil)
-     (239 239 font-lock-variable-name-face)
+     (239 239 font-lock-builtin-face)
      (240 248 nil)
-     (249 249 font-lock-variable-name-face)
+     (249 249 font-lock-builtin-face)
      (250 260 nil)
-     (261 261 font-lock-variable-name-face)
+     (261 261 font-lock-builtin-face)
      (262 265 nil)
      (266 268 font-lock-comment-delimiter-face)
      (269 284 font-lock-comment-face)
      (285 296 font-lock-function-name-face)
      (297 297 nil)
-     (298 298 font-lock-variable-name-face)
+     (298 298 font-lock-builtin-face)
      (299 307 nil)
-     (308 308 font-lock-variable-name-face)
+     (308 308 font-lock-builtin-face)
      (309 309 nil)
      (310 319 font-lock-type-face)
      (320 322 nil)
-     (323 324 font-lock-variable-name-face)
+     (323 324 font-lock-builtin-face)
      (325 325 nil)
      (326 329 font-lock-type-face)
      (330 330 nil)
-     (331 332 font-lock-variable-name-face)
+     (331 332 font-lock-builtin-face)
      (333 335 nil)
      (336 347 font-lock-function-name-face)
      (348 348 nil)
-     (349 349 font-lock-variable-name-face)
+     (349 349 font-lock-builtin-face)
      (350 361 nil)
      (362 364 font-lock-comment-delimiter-face)
      (365 381 font-lock-comment-face)
      (382 384 font-lock-function-name-face)
      (385 385 nil)
-     (386 387 font-lock-variable-name-face)
+     (386 387 font-lock-builtin-face)
      (388 388 nil)
      (389 393 font-lock-type-face)
      (394 394 nil)
      (395 397 font-lock-type-face)
      (398 398 nil)
      (399 401 font-lock-function-name-face)
      (402 402 nil)
-     (403 403 font-lock-variable-name-face)
+     (403 403 font-lock-builtin-face)
      (404 406 nil)
      (407 407 font-lock-type-face)
      (408 414 nil))))