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/src/__tests__/parsers.test.ts

@@ -15,6 +15,7 @@ describe("isLanguageSupported", () => {
     expect(isLanguageSupported("test.java")).toBe(true);
     expect(isLanguageSupported("test.swift")).toBe(true);
     expect(isLanguageSupported("test.dart")).toBe(true);
+    expect(isLanguageSupported("test.cs")).toBe(true);
     expect(isLanguageSupported("test.ex")).toBe(true);
   });
 
@@ -53,6 +54,7 @@ describe("getSupportedExtensions", () => {
     expect(extensions).toContain("rs");
     expect(extensions).toContain("swift");
     expect(extensions).toContain("dart");
+    expect(extensions).toContain("cs");
     expect(extensions).toContain("json");
   });
 });