Add bundled language aliases for JS/TS code fences

[keldenl]

Summary

• Normalize common code-fence aliases using Shiki’s bundled language metadata (js/cjs/mjs, ts/cts/mts, zsh, etc.) in @streamdown/code.
• Add tests covering alias support.

Why

LLMs commonly emit "js" or "ts" fences by default. Mapping Shiki’s built-in aliases to bundled grammars keeps syntax highlighting working without prompt workarounds to force "javascript"/"typescript".

Testing

• pnpm test
• pnpm run check
• pnpm run check-types

Issue links

No existing issues found for alias-based highlighting after broad search (code block, highlight, syntax highlight, alias, js).

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[keldenl]

Addressed the inline review note about casing: normalizeLanguage now lowercases unknown ids (while still mapping Shiki aliases). This keeps case handling consistent for Shiki lookups.