Allow non-block themes to add theme support for border settings

[poena]

Allow themes without theme.json to opt-in to block border tools via:
add_theme_support( 'border' );

Follow up to https://core.trac.wordpress.org/ticket/57460 and https://core.trac.wordpress.org/ticket/57649 with a more limited scope than 'appearance-tools'.

Corresponding Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/51777

[poena]

Updates theme support in wp-includes/class-wp-theme-json-resolver.php

[poena]

Testing instructions

Activate a classic theme.

Open the themes functions.php and locate a setup function or similar that uses add_action( 'after_setup_theme',.For example, if you are using Twenty Twenty-One, look for the function called twenty_twenty_one_setup()

Add add_theme_support( 'border' ); inside that function.

Open the block editor.
Add a block that supports borders, for example, a group.
Confirm that the border controls are available in the block settings sidebar.

Adjust the border width, style, and radius.
Select a custom border color using the color picker. Confirm that the border works in the editor and the front.

• I am suggesting the color picker because themes need to add their own CSS to make the custom theme palette colors work.
• Some themes already style the block borders; it is up to each theme developer to adjust CSS where needed if they want to opt-in to the controls.

[audrasjb]

Before patch: no border styling :((

[audrasjb]

After patch: border styling for the win \o/

[audrasjb]

Thanks for the ticket and patch, this is a great addition for classic themes!
Succesfuly tested on my side. Thanks for the detailed testing instructions.

[audrasjb]

In 55995:

Themes: Allow non-block themes to add theme support for border settings.

This changeset allows themes without theme.json to opt-in to block border tools via add_theme_support( 'border' );.

Follow-up to [55067], [55273].

Props poena, audrasjb.
Fixes #58598.

[stevenlinx]

Added to misc dev note.
draft: ​https://make.wordpress.org/core/?p=106236&preview=1&_ppp=2977223417