fix(typescript) highlight generic function declarations

[Kernel-Hunter]

Fixes #4550

Changes

The function declaration rule comes from the JavaScript grammar and only matches when ( directly follows the name. With type parameters in between, as in function identity<T>(arg: T), nothing matched: the name got no title.function and the arguments weren't scoped as params.

This adds a TypeScript-only variant for function name<, and a mode for the type parameter list that handles nested <...> (T extends Record<string, unknown>) and ignores the > of => inside function types (F extends (x: number) => void). After the list closes, the existing params rule picks up the argument list. Output for non-generic functions and plain JS is unchanged.

Not covered: function* gen<T>() and anonymous function<T>(). I can do those separately if you want them.

Checklist

• Added markup tests (test/markup/typescript/generic-functions)
• Updated the changelog at CHANGES.md
• I have read and followed our AI-assisted contributions policy

Assisted-by: Claude Opus 5.5

[kgeminicdev]

Thanks, this fixes #4550 nicely. Nested generics, defaults, => in function types and declare function all highlight correctly, and plain functions are unchanged. One regression with string literal types in the type parameters, details and a tested fix inline.

[kgeminicdev]

A > inside a string literal type closes the type parameter list early, since strings aren't in contains here. On main these highlight correctly; with this change:

• function f<T extends "a>b">(x: T) {}: the string is cut at >, and ">(x: T) {} is highlighted as a string.
• function f<T extends `${number}>`>(x: T) {}: the template literal is cut, and since template strings span lines, everything after it is highlighted as a string.

Adding the string modes fixed both cases for me, and generic-functions still produces the same output:

contains: [
  { match: /=>/ },
  tsLanguage.exports.CLASS_REFERENCE,
  hljs.APOS_STRING_MODE,
  hljs.QUOTE_STRING_MODE,
  tsLanguage.contains.find(m => m.begin === '`'), // or export TEMPLATE_STRING alongside CLASS_REFERENCE
  'self'
]

Might be worth adding both lines to the markup test.