Allow non-block themes to add theme support for link color settings

[poena]

Allow themes without theme.json to opt-in to block link color tools via:

add_theme_support( 'link-color' );

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/51775

[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( 'link-color' ); inside that function.
Open the block editor.
Add a paragraph with a link.
Open the block settings sidebar and confirm that the link option is available in the color panel.

Select a custom link color using the color picker. Confirm that the color 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)

[audrasjb]

Before patch: no support for link's custom styles

[audrasjb]

After patch: wooohooo \o/

[audrasjb]

Thanks for the ticket and patch Carolina!

Tested on my side using a classic theme: works fine!

Self assigning for commit.

[audrasjb]

In 55996:

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

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

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

Props poena, audrasjb.
Fixes #58597.

[stevenlinx]

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