fix(cpp): remove function.dispatch scope from identifiers that start with a keyword (#4394)

[xxiaoxiong]

Fixes #4394

Changes

• Removed className: 'function.dispatch' from FUNCTION_DISPATCH so that identifiers that look like function calls are no longer highlighted as built_in
• Added word boundary (\b) to each keyword in the negative lookahead so prefixes are properly handled
• Cleared the classNameAliases for function.dispatch accordingly

Context

Previously, identifiers such as testing() were highlighted as built_in by FUNCTION_DISPATCH, while identifiers starting with a registered keyword (e.g., for_this(), whilexyz()) were correctly excluded by the negative lookahead. This produced inconsistent highlighting.

With this fix, FUNCTION_DISPATCH still exists for relevance scoring via _hint keywords, but it no longer applies a visual scope to the matched text, producing consistent unhighlighted identifiers across the board.