Should theme.json allow registration of custom image sizes?

[justintadlock]

More and more, I have had less of a need for functions.php with block themes. Almost every theme feature can be opted into via theme.json. One of the remaining pieces of that is equivalents for set_post_thumbnail_size() and add_image_size().

I was undecided on whether this was worth a ticket yet or if some things should just be left to good ol' PHP. Perhaps there is even a future where end-users could adjust this via the Global Styles system.

I haven't thought this one completely through but wanted to throw it out there to see anyone else had stronger opinions on it yet.