Add category filtering to StartPageOptions patterns modal

[richtabor]

The current requirement for including a pattern in the StartPageOptions modal is to set Block Types: core/post-content. I also prefer to set Post Types: page, which limits these to only populating within "Add new page" flows (not posts).

This gives us a "Choose a pattern" modal, which renders when creating a new page. This is nice, but could be much more useful if we had additional categorization, much like the Pattern Explorer, to filter by page category/type (i.e. about, services, portfolio, etc).

Acceptance criteria

• Categories are added to the StartPageOptions modal, relative to the patterns that have declared a block type of core/post-content. Not every category is rendered, only the ones that have page patterns.
• A pattern with a block type of core/post-content, and a category of about would be available within the StartPageOptions modal in the "About" category. There's no need to redeclare categories, but rather pull from the existing source.

The current StartPageOptions modal:

The current Pattern Explorer:

I would expect we could share the design and structure of the explorer; perhaps merge the UI.

[glendaviesnz]

Seems like a duplicate of #46498 ?

[richtabor]

Seems like a duplicate of #46498 ?

Yea, in part. Good catch. I lean towards keeping this one, as one of the action items noted in that issue.

And it's actionable/updated.