Add block pattern management

@annezazu reached out to see what I thought of integrating my Block Pattern Builder plugin into Create Block Theme. One of the pain points being that the pattern creation process was a bit too manual for non-technical users.

BPB was originally built as a proof of concept of a pattern manager that I'd envisioned core WP eventually handling. The current release is not 100% up to date with the current Patterns API features, but I've been working locally to update it.

I'm happy to start on the process of a PR if this is desirable for the plugin.

If so, I imagine one thing that would need to change is prefixes. Names/prefixes:

• Patterns: bpb/pattern-name
• CPT: bpb_pattern
• Taxonomy: bpb_pattern_category
• Role capabilities: bpb_cap_name

Any thoughts on naming/prefixing for those things?

Potential features:

• Allow user-created patterns to be exported with a theme
• Allow deleting theme patterns via the Editor; or allow theme patterns to be excluded from the theme export

Start by reviewing the linked Block Pattern Builder plugin and the current Patterns API work mentioned in the issue. Clarify whether the goal is full pattern management or only the listed export and deletion capabilities, then resolve the proposed prefixes and define the behavior that would count as complete.