Post blocks: ensure all are able to be transformed to other post blocks

[annezazu]

All post blocks should be replaceable / convert to other blocks. I should be able to easily swap a Date with an Author block, or Categories with Tags. Currently this is inconsistent and incomplete as you can see below in two examples:

Pulled from: #48443

Marking as needs dev since this seems to need some PRs to ensure transforms are possible.

[ntsekouras]

Post terms transforms(categories/tags, etc..) are already in: #48328.

[annezazu]

Awesome! Thanks for noting that.

[carolinan]

I think it would be helpful to have a list of the expected transforms, or is it literally "all post blocks should be able to be transformed into all post blocks"

[mtias]

Some transforms are somewhat semantic (transforming between content and excerpt, or between different types of post meta), but others are really just about swapping a block with another block.

[carolinan]

It is going to be a very long list of blocks in the transform UI, if all post blocks can be transformed to all post blocks:

[mtias]

Not necessarily, the transforms list can be for what was explicitly added, and then there can be a "Replace with ->" that opens another menu or quick inserter with access to all the blocks.