Add theme support for appearance tools

[isabel_brison]

It should be possible for classic themes to opt in to appearance tools support, which enables a few design tools for blocks (complete list here: ​https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-includes/class-wp-theme-json.php#L576)

Previously this was attempted in #57460, and then reverted in #57649 due to block gap support not working correctly. That issue has since been fixed (​https://github.com/WordPress/gutenberg/pull/56130). Further small bugs are tracked in ​https://github.com/WordPress/gutenberg/issues/56131 and well on the way to being fixed.

This seems like a good time to give appearance tools support another try!

Trac ticket: https://core.trac.wordpress.org/ticket/60118

I just copied over the changes from #3846 as that patch was no longer applying cleanly.

Thanks for the reviews folks! All changes have been made.

[isabel_brison]

In 57255:

Themes: Add theme support for appearance tools.

Reapplies the patch reverted in #57649 as the original patch was no longer applying cleanly. Adds theme support for appearance tools to WP_Theme_JSON_Resolver.

Props andrewserong, mukesh27, noisysocks.
Fixes #60118.

Thanks for the reviews folks! Committed in r57255.

[SergeyBiryukov]

In 57256:

Docs: Add a mention of appearance-tools as a possible value for add_theme_support().

Follow-up to [57255].

See #60118.

[isabel_brison]

Thanks for adding that @SergeyBiryukov! I wasn't aware of the list. Looks like it's missing border, block-templates and block-template-parts, possibly others too. Should the list be exhaustive? If so might be worth opening a ticket to make a full inventory.

[SergeyBiryukov]

Replying to isabel_brison:

Should the list be exhaustive? If so might be worth opening a ticket to make a full inventory.

That would be great, yes :)