fix: sync syntax highlighting and keyword table with brainrot language

[leo-aa88]

Summary

• syntaxes/brainrot.tmLanguage.json: remove sus, which was never a real Brainrot keyword (no trace of it in the brainrot lexer, ever); add the missing bet builtin (stdrot/bet.c); fix the builtin-function regex — it was missing the outer grouping parens, so only yapping and the trailing alternative got a \b boundary while baka, yappin, ragequit, chill did not.
• README.md keyword table: grimace → const was stale — that keyword was renamed to deadass upstream a while back. cooked → auto was flat-out wrong — #cooked is an unimplemented preprocessor directive (#include equivalent), not a keyword, and auto isn't a Brainrot keyword at all. Added the missing thicc → long long and rant → string type rows.
• CHANGELOG.md: noted the fix under Unreleased.

Cross-checked against lang.l, lang.y, and docs/ in Brainrotlang/brainrot (source of truth).

Test plan

• python3 -m json.tool syntaxes/brainrot.tmLanguage.json — valid JSON
• Verified the fixed builtin-function regex against a sample snippet using bet, baka, slorp, yapping — all four match correctly with word boundaries
• Manually diffed the keyword table against the brainrot README's keyword table

🤖 Generated with Claude Code

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