consider adding "scope" to block metadata

[jaked]

Some blocks may make sense only in certain contexts, e.g. a block intended to be used in Markdown. We could add metadata to restrict what scope they appear in.

Feedback from @HiDeoo:

• Is there any plan for marking some blocks as only usable directly in Markdown and not be listed in the block picker search bar in the "fullscreen file renderer view"? My block (and I think there may be more) feels relevant embedded in the middle of a markdown file but kinda makes no sense alone in the other view.