Add block instance element color support for buttons and headings

[aaronrobertshaw]

This ticket tracks the backporting of PHP files for the following Gutenberg changes:

​https://github.com/WordPress/gutenberg/pull/53667

These changes provide the ability to specify colors for button and heading elements within a block instance, such as an individual group block.

This is a backport PR for WordPress 6.4 that includes the following PHP Gutenberg changes:

• ​https://github.com/WordPress/gutenberg/pull/53667

It also adds a couple of unit tests to cover the application of the wp-elements-* classname for buttons and headings.

Trac ticket: https://core.trac.wordpress.org/ticket/59309

Thanks for the review @tellthemachines 👍

I suppose it's not worth adding a test for wp_render_elements_support_styles given that it doesn't return anything significant.

I was in two minds about this, on one hand it would be good to confirm that elements styles are generated appropriately, on the other hand it's really just testing the style engine. Ultimately, my reasoning in omitting it was as you suggested, it doesn't really return anything of note.

[isabel_brison]

In 56604:

Editor: add block instance element color support for buttons and headings.

Adds support for buttons and headings to the colors and elements block supports, allowing button and heading element colors to be changed on individual blocks.

Props aaronrobertshaw.
Fixes #59309.

Committed in r56604.