WIP: Implement the webfonts API

[aristath]

In WordPress/wordpress-develop#1736 we're working on a webfonts API for WordPress Core.
This PR is a companion to the core patch, to test the API.

Eventually, the goal is to have webfonts defined in the theme.json file, but in this 1st iteration we're using a wp_register_webfonts function.

[aristath]

Also see WordPress/gutenberg#35591 in the Gutenberg repo regarding implementing the webfonts schema in theme.json.

[aristath]

Once the core patch gets merged and we have an implementation in Gutenberg for theme.json, we should add the below in theme.json, under settings.typography:

"webfonts": [
	{
		"fontFamily": "Source Serif Pro",
		"fontWeight": "200 900",
		"fontStyle": "normal",
		"fontStretch": "normal",
		"src": "file:./assets/fonts/source-serif-pro/SourceSerif4Variable-Roman.ttf.woff2",
		"provider":"local"
	},
	{
		"fontFamily": "Source Serif Pro",
		"fontWeight": "200 900",
		"fontStyle": "italic",
		"fontStretch": "normal",
		"src": "file:./assets/fonts/source-serif-pro/SourceSerif4Variable-Italic.ttf.woff2",
		"provider":"local"
	}
]

[kjellr]

I'm going to close this one for now, since it's outdated and the corresponding PR isn't going to be a part of 5.9.