https://core.trac.wordpress.org/ticket/54336

This PR ports the public functions to interact with the global styles & settings.

This is based on #1808 The only commit this adds is 3a0e7f2

How to test

(same than for #1808)

• Use TwentyTwentyOne.
• In its functions.php file, comment the lines that add support for the following: custom-line-height, experimental-link-color, custom-spacing, custom-units.
• Create a theme.json in the top-level directory of the theme and paste these contents:

{
	"version": 2,
	"settings": {
		"color": {
			"custom": false,
			"link": true
		},
		"spacing": {
			"padding": true
		},
		"typography": {
			"lineHeight": true,
			"fontSizes": []
		},
		"layout": {
			"contentSize": "840px",
			"wideSize": "1100px"
		}
	}
}

In the post editor

Verify that the settings are respected: users can't add custom colors, link color is present, the cover block has the spacing panel with the padding control, the typography panel only shows line-height but not font sizes.

Change the settings and verify that it still works as expected.

Substitute the content of the theme.json by these:

{
	"version": 1,
	"settings": {
		"color": {
			"custom": false,
			"link": true
		},
		"spacing": {
			"customPadding": true
		},
		"typography": {
			"customLineHeight": true,
			"fontSizes": []
		},
		"layout": {
			"contentSize": "840px",
			"wideSize": "1100px"
		}
	}
}

Verify that it still works as expected.

In the front

Verify that there's an embedded stylesheet whose name is global-styles-inline-css and that it contains:

• the CSS Custom Properties for the presets defined by core and the theme
• the layout/alignment styles
• the classes for the presets defined by core and the theme

Add the following styles to the theme.json:

{
        "version": 2,
        "settings": { ... },
	"styles": {
		"color": {
			"background": "black",
			"text": "white"
		}
	}
}

and verify that the block styles are added, in addition to the above: