Allow setting multiple block styles.

[richrd]

Description

This PR allows setting multiple active block styles for a block. Closes #14598 .
To better distinguish between active and inactive styles I added a check mark to the top right corner of the style preview (see screenshot). A similar check mark can be seen in the media library when selecting images.

How has this been tested?

Tested with a fresh wp-env environment. Made sure that setting multiple styles work in the Gutenberg editor and that they are correctly rendered on front end. Passes the entire test suite.

Screenshots

Types of changes

This changes the behavior of block styles so that multiple can be active at once. This doesn't break using just a single style and if no styles are selected the default style is still enforced. However since multiple styles can be selected, the user interaction changes a bit: choosing a style no longer disables the previous selected style.

One thing to consider is that this makes it possible to activate styles that conflict with each other. Most of the time it's probably obvious like setting multiple styles that affect background or border, but maybe not in all cases. Unfortunately I can't see any bulletproof solution to this as discussed here #14598 .

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.