rust: add safe keyword

This is a "weak" keyword, only applicable in Rust 2024+ `unsafe extern` blocks: https://doc.rust-lang.org/reference/keywords.html#r-lex.keywords.weak.safe

Author: fw-immunant

src/languages/rust.js

@@ -318,6 +318,12 @@ export default function(hljs) {
         className: "punctuation",
         begin: '->'
       },
+      {
+        className: 'unsafe_extern',
+        begin: /unsafe\s+extern/,
+        end: /\}/,
+        keywords: [ 'safe' ],
+      },
       FUNCTION_INVOKE
     ]
   };