Reduce Shiki loading with the JavaScript regex engine

[ruibaby]

What changed

• add a custom Shiki bundle backed by the JavaScript regular expression engine
• route both the Tiptap code block extension and rendered comment highlighting through the custom bundle
• bundle only the GitHub Dark theme while keeping built-in languages lazy-loaded
• add tests for syntax highlighting and the Tiptap extension compatibility surface

Why

The default Shiki bundle uses the WebAssembly-based Oniguruma engine. Opening the comment editor or rendering a comment containing code previously required loading an approximately 232 kB gzip WASM chunk.

The custom bundle removes the Oniguruma dependency and reduces the editor Shiki loading path from approximately 306.8 kB to 95.7 kB gzip.

Validation

• pnpm -C packages/comment-widget test
• pnpm exec tsc -p packages/comment-widget/tsconfig.json --noEmit
• pnpm exec biome check package.json packages/comment-widget/vite.config.ts packages/comment-widget/src/comment-content.ts packages/comment-widget/src/shiki-bundle.ts packages/comment-widget/tests/shiki-bundle.test.ts
• pnpm build
• verified that emitted runtime chunks contain neither WebAssembly.instantiate nor the Oniguruma engine