Webfonts API: Document fontFace and it’s values in theme.json schema

[grappler]

What?

Document the fontFace properties that we introduced in WordPress 6.0 as part of the WebFonts API.

Why?

Because there is no documentation.

How?

By including the basic technical documentation to the theme.json schema.

Testing Instructions

Create a theme.json and paste the following in the code. Currently, it says that the fontFace is not allowed. Replace https://schemas.wp.org/trunk/theme.json with https://raw.githubusercontent.com/grappler/gutenberg/schema-fontFace/schemas/json/theme.json to test changes.

`theme.json` code

{
	"$schema": "https://schemas.wp.org/trunk/theme.json",
	"version": 2,
	"settings": {
		"typography": {
			"fontFamilies": [
				{
					"fontFamily": "\"Source Serif Pro\", serif",
					"name": "Source Serif Pro",
					"slug": "source-serif-pro"
				},
				{
					"fontFamily": "\"Inter\", sans-serif",
					"name": "Inter",
					"slug": "inter",
					"fontFace": [
						{
							"fontFamily": "Inter",
							"fontWeight": "200 900",
							"fontStyle": "normal",
							"fontStretch": "normal",
							"src": [ "file:./assets/fonts/inter/Inter.ttf" ]
						}
					]
				}
			]
		}
	}
}

Screenshots

[fabiankaegy]

This is a great addition :) Added one comment. But that can also happen in a follow up 👍

[grappler]

It would be great if this could be backported to wp/6.0 too. Happy to create a second PR for that.

[Mamaduka]

@adamziel, I think this needs to be backported so that the schema URL is correctly resolved. It's not a blocker for a minor release.

[adamziel]

I just cherry-picked this PR to the wp/6.0 branch to get it included in the next release: 6a79add