Background block supports: add support for top-level styles in theme.json

[ramonjd]

Syncing Gutenberg PRs:

• Global styles: background UI controls gutenberg#59454
• Global styles: add background image to top-level theme.json styles gutenberg#59354
• Background block supports: move block support defaults to gutenberg_render_background_support and revert gutenberg_get_background_support_styles gutenberg#59889
• Global styles: force root min-height of 100% for backgrounds gutenberg#59809
• Background images: remove required "file" prop gutenberg#61387
• Background images: remove lingering "file" prop gutenberg#61469

Trac ticket: https://core.trac.wordpress.org/ticket/61123

How to test

This PR enables top-level background images. You can:

1. Add a site background image in theme.json:

{
	"$schema": "https://schemas.wp.org/trunk/theme.json",
	"version": 2,
	"settings": {
		"appearanceTools": true
	},
	"styles": {
		"background": {
			"backgroundImage": {
				"url": "https://images.pexels.com/photos/22484288/pexels-photo-22484288/free-photo-of-the-circular-stone-terraces-of-the-inca-ruins.jpeg"
			},
                        "backgroundSize": "cover"
		}
	}
}

2. Or upload/add your own background image in the site editor global styles sidebar.

2024-05-28.12.11.42.mp4

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[noisysocks]

Took this branch for a spin without the Gutenberg plugin activated. I tested setting a background image via Global Styles on TT4 and creating a new theme with Create Block Theme, setting a background via Global Styles, and then saving those changes to theme.json. All worked as expected.

LGTM but maybe fix up that ?? since I think we're not supposed to use them. (I can't actually remember.)

[noisysocks]

Looking good 👍 – do you need me to commit?

[ramonjd]

Looking good 👍 – do you need me to commit?

Thank you!! Yes please! I have pleb rights.

[noisysocks]

Committed in r58222.