types: export additional union types used internally #1818
Description
This issue comes out of #1227 and has one example of the suggestion to export certain union types as their own standalone type in the draft PR #1228 (in this commit specifically).
Seems like a no brainer! We should export - and reuse - different kinds of union types we use internally. This is more of a refactoring task and so could be a good first issue to pick up. This commit shows off the proposed solution well. It is particularly applicable to the elements array on Blocks, which can contain a certain subset of different Block Elements.
Technically the refactor could be a patch version bump and exporting the new types would be a minor version bump. As such, this could go out in a patch or minor new release in the 2.x line of types.