Add support for custom Shiki languages

[amadeus]

A quick side quest for #301

Essentially this re-creates a language registry for custom shiki grammars. I haven't been really tested this yet though, but I think it should just work.

Basically I created a registration API like custom theme where you provide a language, a loader and some file extensions/filenames to map the language to. Then just rely on automatic language detection or utilize language overrides:

registerCustomLanguage(
  'my-lang', 
  () => import('./my-lang.tmLanguage.json'),
  ['mylang']
);

// And later on you would just do something like

<File
  file={{
    name: 'example.mylang',
    contents: '' // etc
  }}
/>

// or
<File
  file={{
    name: 'example',
    contents: '', // etc
    lang: 'my-lang'
  }}
/>

Fixes #301

[vercel]

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Project	Deployment	Review	Updated (UTC)
pierrejs-diff-demo	Ready	Preview	Jan 29, 2026 7:34am
pierrejs-docs	Ready	Preview	Jan 29, 2026 7:34am
pierrejs-solid-diff-demo	Ready	Preview	Jan 29, 2026 7:34am

[Copilot]

Pull request overview

Adds an API to register and use custom Shiki languages (dynamic loaders + filename/extension mapping) so syntax highlighting can work for non-bundled grammars.

Changes:

• Introduces registerCustomLanguage and a custom language registry used by language resolution.
• Extends filename/extension-to-language detection to support custom mappings.
• Updates public types/exports and docs to reflect custom language support.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
packages/diffs/src/utils/setLanguageOverride.ts	Updates typing for language overrides (now via overloads).
packages/diffs/src/utils/getFiletypeFromFileName.ts	Adds a custom mapping Map and checks it during language inference.
packages/diffs/src/types.ts	Expands SupportedLanguages to allow custom string language IDs.
packages/diffs/src/index.ts	Exports the new registerCustomLanguage API.
packages/diffs/src/highlighter/languages/resolveLanguage.ts	Resolves languages from either bundled Shiki loaders or registered custom loaders.
packages/diffs/src/highlighter/languages/registerCustomLanguage.ts	New API to register custom language loaders and optional extension/filename mappings.
packages/diffs/src/highlighter/languages/constants.ts	Adds storage for registered custom language loaders.
apps/docs/app/docs/page.tsx	Wires the new utility example into the docs page rendering/preloading.
apps/docs/app/docs/Utilities/content.mdx	Adds documentation section for registerCustomLanguage.
apps/docs/app/docs/Utilities/constants.ts	Adds a runnable docs helper snippet demonstrating registerCustomLanguage.

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