Section Styles: Enable default block styles to be defined using styles/*.json files

[bacoords]

What problem does this address?

Theme.json can get rather large/monolithic, the ability to use smaller JSON files (as introduced for theme and block style variations) is leading us towards a more well-organized and composable future for themes. But adding all of your block styles into one theme.json file is still seen as a hurdle for theme builders.

What is your proposed solution?

Right now we can set up block style variations using separate JSON files in /styles/*.json. We should extend this to allow developers to define the default styles for any block using this same approach.

One example could just be support for using

	"title": "Default",
	"slug": "default",

which would be recognized immediately as additional default style choices for blocks.

See also: #38890 (comment)