Block Styles in Theme.json: Add support for elements/interactive states and custom CSS

[scruffian]

What problem does this address?

When block styles are defined in theme json, it's not possible to control:

• elements
• hover /active/ focus
• custom css.

What is your proposed solution?

styles > blocks > block name > variations > variation name should support the same things as blocks:

• elements
• hover /active/ focus
• custom css.

[carolinan]

Adding custom CSS was explored here,
#49396 but was made difficult:

1. Because of how the "default" block style can be any registered block style variation. Please read the comments here:
   Add custom CSS support to elements and block style variations #49396 (review)

2. Lack of space and design for the controls in the Styles sidebar in the Site Editor. Both for elements and block styles.