Move block style variation CSS to theme.json

[justintadlock]

Currently, nearly every style variation in the theme requires defining CSS properties via settings.blocks.blockName.custom, which are applied via block stylesheets. This is so that they can be overwritten via JSON in global style variations or child themes.

There are major limitations with this method, namely that the CSS properties are loaded on every page, even when they are unneeded.

Requires:

• Block Styles: Extend block style variations as mechanism for achieving section styling WordPress/gutenberg#57908