Add Pattern Directory support to the core editor about pattern-directory

[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 (#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 (#3)
• Update GB to use this endpoint to load core patterns (server-side) WordPress/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.