Introduce singular capabilities for managing individual themes

[johnbillion]

As we did in #35614 for taxonomy terms, singular capabilities should be introduced for switching, editing, deleting, and updating individual themes.

This would allow fine-grained cap checks such as current_user_can( 'switch_theme', $theme ) and current_user_can( 'delete_theme', $theme ).

[flixos90]

All of these are part of a larger goal.