Add support for global styles custom css

[glendaviesnz]

In the Gutenberg repo, a feature has been merged that allows custom CSS to be added to Global Styles via the style editor, in a similar way to how the Customizer allows the addition of custom CSS.

In order to support this a number of PHP file changes need to be ported from the Gutenberg plugin.

For further context, the Gutenberg PR that introduced this feature is: ​https://github.com/WordPress/gutenberg/pull/46141/

Trac ticket: https://core.trac.wordpress.org/ticket/57536

Still draft as need to look at also bringing across changes from ​https://github.com/WordPress/wordpress-develop/pull/3896, and also adding some tests

@aristath - I have updated this to match the changes merged in with ​https://github.com/WordPress/gutenberg/pull/47396, which will affect ​https://github.com/WordPress/wordpress-develop/pull/3925 also

@Mamaduka have pulled across ​https://github.com/WordPress/gutenberg/pull/47062, but some unit tests are failing on multisite which may be related to this and I ran out of time to work out why - will take another look tomorrow if you don't have time to look at it.

@glendaviesnz, I was able to resolve failing multisite tests by granting capabilities to admin users.

I added the following to WP_REST_Global_Styles_Controller_Test::wpSetupBeforeClass

{{{php
if ( is_multisite() ) {

grant_super_admin( self::$admin_id );

}
}}}

@glendaviesnz, we should probably place the grant_super_admin part of the code after $admin_id is assigned.

[hellofromTonya]

Changes are requested for the patch ​PR 3896.

[hellofromTonya]

Adding performance keyword for caching added into wp_get_global_styles_custom_css(). cc @flixos90.

Thanks @felixarntz and @hellofromtonya for the detailed feedback!

@hellofromtonya I have run the suggested tests, re load-styles.php:

Step 1: Added the following constants to wp-config.php:

define( 'CONCATENATE_SCRIPTS', true );
define( 'WP_DEBUG', false );
define( 'SCRIPT_DEBUG', false );

Step 2: Applied this PR
Step 3: Started localhost environment with

npm install
npm run build:dev
npm run env:start
npm run env:install

Step 4: Logged in and opened the WP Admin area.

✅No fatal error occured
✅The load-styles.css stylesheet loaded
✅Navigated through screens and no fatal error occured

I think (correct me if I'm wrong) this PR is good shape for commit.

• The concern @felixarntz and I had has been tested to validate it's not a concern ✅
• All feedback (minus a too long to read inline comment request) has been addressed 💹
• Open questions have been addressed ✅

Is there anything else pending before commit? @glendaviesnz @dream-encode @felixarntz?
If no, could the committers please approve for confirmation? 🙇

[flixos90]

Planning to commit this shortly.

[flixos90]

In 55192:

Editor: Add support for custom CSS in global styles.

This changeset introduces functions wp_get_global_styles_custom_css() and wp_enqueue_global_styles_custom_css(), which allow accessing and enqueuing custom CSS added via global styles.

Custom CSS via global styles is handled separately from custom CSS via the Customizer. If a site uses both features, the custom CSS from both sources will be loaded. The global styles custom CSS is then loaded after the Customizer custom CSS, so if there are any conflicts between the rules, the global styles take precedence.

Similarly to e.g. [55185], the result is cached in a non-persistent cache, except when WP_DEBUG is on to avoid interrupting the theme developer's workflow.

Props glendaviesnz, oandregal, ntsekouras, mamaduka, davidbaumwald, hellofromtonya, flixos90.
Fixes #57536.

Committed in https://core.trac.wordpress.org/changeset/55192

Thanks @felixarntz, @hellofromtonya and @dream-encode !