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": {

packages/highlight/src/__tests__/highlighter.test.ts

@@ -62,6 +62,19 @@ describe("highlightCode", () => {
     expect(stringToken?.style).toBe("string");
   });
 
+  it("highlights C# code", () => {
+    const code = 'class Greeter {\n    string message = "hello";\n}';
+    const result = highlightCode(code, "test.cs");
+
+    expect(result).toHaveLength(3);
+
+    const classToken = result[0].find((t) => t.text === "class");
+    expect(classToken?.style).toBe("keyword");
+
+    const stringToken = result[1].find((t) => t.text.includes("hello"));
+    expect(stringToken?.style).toBe("string");
+  });
+
   it("highlights TSX code with correct dialect", () => {
     const code = 'const el = <div className="test">hello</div>;';
     const result = highlightCode(code, "test.tsx");

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");
   });
 });

packages/highlight/src/parsers.ts

@@ -14,6 +14,7 @@ import { parser as phpParser } from "@lezer/php";
 import { parser as rustParser } from "@lezer/rust";
 import { parser as xmlParser } from "@lezer/xml";
 import { parser as yamlParser } from "@lezer/yaml";
+import { csharpLanguage } from "@replit/codemirror-lang-csharp";
 import { parser as elixirParser } from "lezer-elixir";
 import type { Parser } from "@lezer/common";
 
@@ -62,6 +63,8 @@ const parsersByExtension: Record<string, Parser> = {
   swift: StreamLanguage.define(swift).parser,
   // Dart
   dart: StreamLanguage.define(dart).parser,
+  // C#
+  cs: csharpLanguage.parser,
   // Elixir
   ex: elixirParser,
   exs: elixirParser,