rust: add safe keyword

This is a "weak" keyword, only applicable in Rust 2024+ `unsafe extern` blocks,
immediately prior to the "extern" keyword:

https://doc.rust-lang.org/reference/keywords.html#r-lex.keywords.weak.safe

Author: fw-immunant

src/languages/rust.js

@@ -230,6 +230,17 @@ export default function(hljs) {
         ],
         relevance: 0
       },
+      {
+        begin: [
+          /\bsafe/,
+          /\s+/,
+          /extern/,
+        ],
+        scope: {
+          1: "keyword",
+          3: "keyword",
+        }
+      },
       {
         begin: [
           /fn/,