CPTs: Disable a post type's toggle if it's supported by the theme

[tyxla]

As discussed in #13568, right now if a post type is supported by the current theme, we shouldn't be able to disable it from settings. So in that case we should remove the toggle of the corresponding post type completely, and replace it with a green checkmark.

First attempt to handle this one was in #14574 (and some of the discussion happened there).

To be precise with the solution, we'll need to be able to retrieve theme supported features (the ones that are registered with add_theme_support().

We could also improve the copy a bit (no need to say "if your theme doesn't support" if we know for sure whether the theme supports the post type already).

[tyxla]

This one requires a new endpoint to retrieve the current features that are supported by the theme, and the Redux infrastructure for that endpoint. With the thought it's more a feature than a last-minute tweak, I'm slating this for v2.

[github-actions]

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[tyxla]

Still valid.