Add global styles variables in relevant places

[johnstonphilip]

With global styles coming to WP Core, we'll likely want to consider adding the global styles variables to Atomic Blocks. Here's an example of that happening for core blocks:

https://github.com/WordPress/gutenberg/pull/20273/files

I'm not sure if this is the right time to start work on this yet, but I just wanted to get it logged for consideration!

[nickcernis]

Noting that the Global Styles proposal does not yet provide a way for Atomic Blocks to add its own variables to the global styles tree. (AB could use WP core style variables, but not add its own.)

I made a proposal with that in mind: WordPress/gutenberg#20449