Backport: Outline support for blocks in theme.json with unit test

[hellofromtonya]

Refresh of PR #3788: adds test commit (that fails before the backport) and then adds the backport commit, while ensuring it's based on trunk.

This PR backports the changes in #43526 that add support for the outline property on theme.json

Trac ticket: https://core.trac.wordpress.org/ticket/57354

Props: @MaggieCabrera and @audrasjb (from the previous PR)

How to Test

Open TT3's theme.json and go to "elements" on about line 596 and then "button" on the next line. Then make the following changes:

• Add this code as the first entry (before "border"):

"outline": {
	"offset": "3px",
	"width": "3px",
	"style": "dashed",
	"color": "red"
},

and in ":hover", add this code as the last entry (after "color"):

"outline": {
	"offset": "3px",
	"width": "3px",
	"style": "solid",
	"color": "blue"
}

Then add a new post and a button block. In the front-end it should render similar to this:

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[audrasjb]

Perfect! Thanks for putting together this PR Tonya!

Just a small note: the margin-* and padding-* properties are listed in the same order they appear when listed on the padding and margin short notation: top, right, bottom, left.

For the sake of consistency, should we order the outline-* accordingly? width, style, color, then offset.

[audrasjb]

Hmm nevermind: this is not what we did for the border-* properties.

[hellofromtonya]

Thanks @audrasjb! Prepping commit right now.

[hellofromtonya]

Committed in https://core.trac.wordpress.org/changeset/55008.