Stub Core endpoint about pattern-directory

[ryelle]

Not a formalized schema, but these are the assumptions I've been working with:

• An index endpoint that returns all core patterns
• (Eventually) a search endpoint returning matching patterns
• (Eventually) an endpoint returning patterns in a given category

The response should be an array of objects, each pattern should match:

• name: string slug
• title: string, human-friendly, possibly translated
• content: string with blocks & HTML, possibly translated
• viewportWidth: int
• categories: array of strings

Categories are core-defined, but plugins can add specific categories… for the start I think we can restrict to the core list ("buttons, columns, gallery, header, text"). These could also be translated, but the slugs in the API should remain the same

[iandunn]

Rough draft at WordPress/gutenberg#26578

[iandunn]

This was just created so we'd have something in the Project, WordPress/gutenberg#26578 is the main PR.

Closing since we can just use a "note" in the Project to track instead.