Make the global stylesheet available for all themes

[oandregal]

This issue is to describe why and how to make the global stylesheet available for all themes.

Why

• We avoid shipping CSS twice. Some CSS that is now bundled with the block-library will live in the global stylesheet. See Only enqueue default font-sizes once #34006 Only enqueue default colors & gradients once #34138
• We can absorb the block CSS into the merging algorithm. This CSS needs to be bundled always Prototype: merge block CSS with theme.json styles #34180 (comment)

How

• If the theme declares a theme.json it works as today.

• If the theme doesn't declare a theme.json we will still enqueue some CSS: the core presets (needed by patterns) and the block styles defined via this new API. These are styles that we are now enqueuing via the block-library anyway. It's not a change of behavior, it's a change in the handle that contains these styles. We shouldn't enqueue the own theme presets declared in functions.php, though: themes that don't have a theme.json are supposed to do this themselves, and enqueuing ours can conflict with theme's styles.

[oandregal]

I have a PR for this at #34334 in case anyone is able to provide a review.