fix(go) highlight the name of a method with a receiver

[alliasgher]

Fixes #4196.

Describe the issue

func (c *exec.Cmd) Run() error gets no hljs-title on Run, while a plain func deferFunc(err *error) highlights fine.

The function mode contains a params sub-mode with endsParent: true, so the first parenthesised group ends the whole function mode. For a method, that first group is the receiver, so the mode terminates at the receiver's ) and TITLE_MODE never gets as far as the method name.

The fix

A separate receiver mode, placed before TITLE_MODE, which does not end its parent:

const RECEIVER = hljs.inherit(PARAMS, {
  begin: regex.concat(/\(/, regex.lookahead(/[^)]+\)\s*(?!func\b)[a-zA-Z_]\w*\s*\(/)),
  endsParent: false
});

I kept the grammar deliberately narrow rather than loosening func generally, since widening it risks auto-detection. The lookahead only fires for a non-empty parenthesised group that is directly followed by an identifier and another open paren, which is the method-declaration shape. A plain function's argument list is not followed by name(, so it never matches and behaves exactly as before.

The (?!func\b) is there for func middleware(h Handler) func(int32) int32: a return type can be followed by (, and without the guard that argument list would be misread as a receiver.

Testing

New markup test test/markup/go/methods.txt covering a pointer receiver, a value receiver, an anonymous receiver (*sync.Mutex), a generic receiver (s *Stack[T]), multiple return values, and the func-typed return above as a negative case. It fails on main and passes with this change.

Full suite: 1594 passing, and no existing test/markup/go/*.expect.txt changed, which is the check that plain functions are untouched. npm run lint and npm run lint-languages are clean.

Two limitations I did not try to fix, both pre-existing and equally true of plain functions today:

• a method's own parameter list is now inside the params mode, so a string in a parameter default/array-length expression loses hljs-string — this makes methods consistent with plain functions rather than introducing a new inconsistency
• methods with non-ASCII names are still not highlighted, since TITLE_MODE is ASCII-only

Checklist

• Added markup tests
• Updated the changelog at CHANGES.md