Global Styles: Add block-level textAlign UI

[t-hamano]

What problem does this address?

#59531 adds textAlign as block support. This allows developers to opt-in a dropdown for textAlign on the block toolbar via code like the following:

{
	"$schema": "https://schemas.wp.org/trunk/block.json",
	"apiVersion": 3,
	"name": "core/test",
	"title": "Test",
	"supports": {
		"typography": {
			"textAlign": true
	}
}

Additionally, you can apply default styles to each block via theme.json.

{
	"version": 2,
	"styles": {
		"blocks": {
			"core/media-text": {
				"typography": {
					"textAlign": "center"
				}
			}
		}
	}
}

However, there is no way to override the default style via the Global Styles UI.

What is your proposed solution?

Add block-level global styles UI. One design idea is as follows.

[t-hamano]

This UI will need to ship to WP6.6. See this comment.

[colorful-tones]

@t-hamano I happened to notice you’re tackling some text-align stuff and I wonder if you think some of your work might solve #60838 🤔 (perhaps it is even a duplicate issue?)

[t-hamano]

#60838 seems to suggest adding Justification controls that already exist in the block toolbar.

I believe this is a separate issue since this issue is intended to add a new text alignment UI.