Adds the at-rule feature (#3449)

Co-authored-by: Patrick Brosset <[email protected]>
Co-authored-by: Daniel D. Beck <[email protected]>

Author: 3 people

features/supports-at-rule.yml

@@ -0,0 +1,4 @@
+name: at-rule()
+description: "The `at-rule()` function, when used with `@supports`, checks if a CSS at-rule is supported. For example `@supports at-rule(@starting-style)` checks if the `@starting-style` at-rule is supported."
+spec: https://drafts.csswg.org/css-conditional-5/#typedef-supports-at-rule-fn
+group: css

features/supports-at-rule.yml.dist

@@ -0,0 +1,6 @@
+# Generated from: supports-at-rule.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support: {}