Add support for border radius presets

[aaronrobertshaw]

This ticket tracks the backport of PHP files for the following Gutenberg updates:

​https://github.com/WordPress/gutenberg/pull/67544

These updates including the border radius settings definition for theme.json as well as configuration for the style engines generation of CSS vars.

Further background and context is available within the original issue:

​https://github.com/WordPress/gutenberg/issues/64041

Trac ticket: https://core.trac.wordpress.org/ticket/63799#ticket

This PR brings the changes from the following Gutenberg PRs to core:

​https://github.com/WordPress/gutenberg/pull/67544

Changes include:

• definitions of presets settings for theme.json
• CSS var settings for style engine

[rollybueno]

We need to wait for ​https://github.com/WordPress/gutenberg/pull/67544 review approval and merging first.

This PR builds on what was proposed in #9406, but adds tests and fixes up an issue where the former PR didn't yet update the valid settings array.

Trac ticket: https://core.trac.wordpress.org/ticket/63799#ticket

This PR brings the changes from the following Gutenberg PR to core: ​https://github.com/WordPress/gutenberg/pull/67544 for support for border radius presets.

Changes include:

• definitions of presets settings for theme.json
• CSS var settings for style engine

Thanks for putting this PR together! While you're not around this week, I've opened up an alternate PR building on this one over in #10113 (it does the same but just adds some tests). I'll close this PR out for now and we can look at merging in #10113 for 6.9.

Thanks folks! I'll commit this tomorrow 👍

[andrewserong]

In 60892:

Editor: introduce border.radiusSizes support in theme.json.

Adds site front end rendering logic for the border.radiusSizes border radius presets, as well as the required logic in WP_Theme_JSON and the style engine.

Props aaronrobertshaw, youknowriad, ramonopoly, poena, joen, jameskoster, richtabor, asafm7, fabiankaegy
Fixes #63799.