Add Pattern Directory support to the core editor

[tellyworth]

The core editor should allow users to search and insert block patterns from the pattern directory, in a similar way to the block directory.

Tentative assumptions:

• Unlike block plugins there is probably no "install" step needed. Patterns from the directory can simply be inserted into a post exactly as core patterns work at present.
• Core patterns might be moved to the Pattern Directory, so they can be improved independent of the core release schedule.
• A local REST API endpoint for searching the pattern directory might be needed, in a similar manner to the block directory endpoint. (Stub Core endpoint #3)
• Patterns from the directory can probably be listed in the same UI as core patterns are at present.

This is an umbrella task, specific issues should be created as needed.

[ryelle]

I un-checklisted the assumption list since they're not actions. From here, I see the action(s) as

• Create a wp.org endpoint that will serve patterns (Stub basic post type and API endpoint #6)
• Import core patterns into the CPT on wp.org
• Deprecate and/or Remove hardcoded core patterns from Core? back-compat concerns?
• Create a local endpoint (in Gutenberg) which will query the wp.org endpoint, like block directory endpoint (Stub Core endpoint #3)
• Update GB to use this endpoint to load core patterns (server-side) Block Patterns: Load patterns from wordpress.org API gutenberg#28800
• Add functionality (state/actions/etc) to dynamically load patterns client-side
• Add support for searching the API & loading pattern results

In past conversations we haven't discussed changing the patterns UI, only the backend data source, so I think that's still a safe assumption.

[iandunn]

The Core patterns are all imported into the CPT now. I left the box above unchecked since we haven't removed them from Core yet.

Although, I'd be tempted to just mark the wp-includes/block-patterns/ as deprecated, but leave them permanently 🤔

Removing them would cause fatals for any plugins that are requreing them.

[iandunn]

I separated importing and removing into separate checklist items, so the checklist can reflect what's done and what's not.