Prepare WP_Theme_JSON_Data class for backporting

[oandregal]

What

This PR follows-up #44015 addressing some post-merge feedback for improving code quality and make it easier to backport.

How

• Class renamed from WP_Theme_JSON_Data to WP_Theme_JSON_Data_Gutenberg.
• Document the new filters inline.
• Create specific temp variables for the filters (input/output) and don't overload the existing.

How to test

Make sure it still works as before.

Use the filters to modify some of the data. For example, this is how the theme data can be modified:

<?php

function filter_global_styles_theme( $theme_json ){
        $new_data = array(
                'version'  => 2,
                'settings' => array(
                        'color' => array(
                                'text'  => false, /* disable text color UI */
                                'palette'    => array( /* New palette */
                                        array(
                                                'slug'  => 'foreground',
                                                'color' => 'red',
                                                'name'  => 'Foreground',
                                        ),
                                ),
                        ),
                ),
        );
        return $theme_json->update_with( $new_data );
}
add_filter( 'global_styles_theme', 'filter_global_styles_theme' );

[gziolo]

Thank you for a quick follow-up 💯

[oandregal]

This is being backported as part of WordPress/wordpress-develop#3247