Hinted at in #34574.

It's time to move towards introducing some new elements to the Elements API. Captions are used across blocks (images, embeds, videos, etc) and a prime example of what we have called elements in the global styles' sense. Right now core registers some default styles for captions through wp-block-styles, which we should transition to this system.

Captions would become a new element alongside Links in the interface, and can be defined globally or per block type.

At the block level, we should probably introduce a new component to encapsulate the definition, class, CSS variables, etc. Blocks can use this component like they use RichText (it'd be a wrapper around RichText anyways).