Backport appearance tools theme_support

[ockham]

Backport of WordPress/gutenberg#43337.

Quoting that PR's description:

What?

Allow a theme to enable appearanceTools to any theme including classic themes.

Why?

Appearance tools are options in the block editor and don't require the use of the FSE to take advantage of. However there's no way to enable those tools for themes that don't use theme.json.

How?

This checks for the 'appearance-tools' support and if present flags settings.appearanceTools as true in the theme portion of Global Styles.

Testing Instructions

Using a theme that does NOT use a theme.json file enable the 'appearanceTools` theme support.

add_theme_support( 'appearance-tools' );

Add a group block to a new post. Note the block settings panel includes Border settings.

Trac ticket: https://core.trac.wordpress.org/ticket/56467
Trac ticket: https://core.trac.wordpress.org/ticket/56487

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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.

[hellofromtonya]

Hmm, this is marked as a Feature in the original Gutenberg PR and an enhancement in the Trac ticket (which is not in the 6.1 milestone). Sadly, this one missed the 6.1 feature freeze deadline.

Could it be reclassified? Deferring that decision to 6.1 Core Tech Leads @jeffpaul @dream-encode @getsource.

[dream-encode]

From Trac:
@hellofromTonya Thanks for the ping. In reading the discussion above it seems that testing is needed for each theme before adding the support. I really respect @poena's opinion as she's much more plugged in to FSE development.
Moving this to 6.2.

[ockham]

Thank you @dream-encode! I'll move the PR to "Bumped to 6.2" on our project board 👍

[ockham]

@hellofromtonya @azaozz @ironprogrammer Here's another PR that didn't make the cut for WP 6.1 -- for y'all's consideration for (early) inclusion in 6.2 😊

[carolinan]

The earlier, the more time there is to test using the theme support in the bundled themes.

[ironprogrammer]

@ockham In #3846 I've added a unit test for this support, and opened an early backport ticket in Trac 57460. ("Early" as in before 6.2 crunch time 😅)

[ockham]

Perfect, thank you very much @ironprogrammer! Closing this in favor of #3846 (which I see has already been merged 🎉)