fix: remove obsolete Flix grammar

[iamdhrv]

Resolves #4440

Changes

• Remove the obsolete built-in Flix grammar.
• Remove its coupled markup and detection fixtures.
• Keep the supported-languages entry pointing to the maintained third-party package.

Checklist

• Markup tests do not apply because this removes the built-in grammar and its existing fixtures.
• Updated the changelog at CHANGES.md.

Tests: npm run build, npm test.

[github-actions]

Build Size Report

Changes to minified artifacts in /build, after gzip compression.

2 Removed Files

View Changes

file	size
es/languages/flix.min.js	-471 B
languages/flix.min.js	-474 B

2 files changed

Total change -2 B

View Changes

file	base	pr	diff
es/core.min.js	8.24 KB	8.24 KB	-1 B
es/highlight.min.js	8.24 KB	8.24 KB	-1 B

[Baltsat]

I checked the removal path against current main. The built-in Flix source and coupled fixtures are still present, while SUPPORTED_LANGUAGES.md keeps the maintained third-party highlightjs-flix path. The build loader discovers core grammars from src/languages/*.js and third-party grammars separately, so the five-file deletion is scoped consistently.

The original 18 checks are green, but GitHub now reports this head as conflicting with main, so those runs do not prove the current base. I found no source-level blocker after a rebase; the remaining step is to resolve the conflict and rerun CI.