feat: add C# syntax highlighting (#1651)

Wire the @replit/codemirror-lang-csharp Lezer grammar into @getpaseo/highlight following the existing per-language pattern (Java/Rust/Swift). Map the cs extension to the parser and alias the csharp and c# markdown fence names so fenced code blocks highlight.

Register csharpLanguage.parser rather than the package's raw parser export: the highlight styleTags are applied only inside csharpLanguage (via parser.configure), so the raw parser would parse but render unstyled.

Closes #1527

Co-authored-by: Clemens Wagner <wagner.clemens@gmx.de>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/highlight/package.json

@@ -44,6 +44,7 @@
     "@lezer/rust": "^1.0.2",
     "@lezer/xml": "^1.0.6",
     "@lezer/yaml": "^1.0.4",
+    "@replit/codemirror-lang-csharp": "^6.2.0",
     "lezer-elixir": "^1.1.2"
   },
   "devDependencies": {