Theme.json Schema: add support for button elements

[mikachan]

What?

In #40260, we added support for button elements to theme.json. We now need to update the schema for theme.json to reflect this accurately.

Why?

This improves the developer experience by automatically validating the contents of theme.json.

How?

By including button as one of the properties listed under the elements section.

I copied the way we're already doing this for core/button here, which has worked in my testing, but I'm not sure if there's a better/more correct way to handle this.

Testing Instructions

1. Activate a theme which you can modify locally.
2. Open the theme's theme.json file and update the $schema to point to your local Gutenberg version (follow the instructions from the docs here).
3. Add some button rules under elements.button and check your IDE doesn't complain. Here is an example:

"elements": {
	"button": {
		"border": {
			"radius": "0",
			"width": "0"
		},
		"color": {
			"background": "black",
			"text": "white"
		},
		"typography": {
			"textTransform": "uppercase"
		},
		"spacing": {
			"padding": {
				"top": "10px",
				"bottom": "10px",
				"left": "20px",
				"right": "20px"
			}
		}
	}
}

4. Add some invalid button properties and check that it does complain (e.g. textDecoration).

[pbking]

Looks and works right to me.

Gave me an opportunity to deep-dive into the schema. :)

[getdave]

LGTM