Convert image caption to inner block

[cbirdsong]

Right now, the image caption is part of the image block. It would be nice if the caption was a specialized inner block similar to list items so that it could have block styles registered on it. This might also make it easier to address #19975.

Edit: It looks like this was already tried in a PR, but that PR has no associated issue.

[aaronrobertshaw]

There is a closed issue (#28920) relating to potentially converting an image block's caption to an inner block.

The possibility was also discussed on several of the PRs refactoring the Gallery block to use inner image blocks. There didn't seem to be much appetite for the change. Since then, caption element styling has been added to global styles which might reduce the potential need for the inner block as well.

Relating to the text alignment for image captions in #19975, there is also some further discussion on #12997.

[creativecoder]

I came across this while working with a user this week--they were looking for a way to style image and video captions and couldn't find one.