Editor: Add a description key to theme.json style variations.

This changeset adds `description` to `VALID_TOP_LEVEL_KEYS` in `class-wp-theme-json.php` to allow for accessibility improvements. The description is added to 
the ARIA label of the style variation container. If the style variation has no description, only the title is used. One purpose is to use the text description 
to improve the style variation preview in the WordPress.org theme directory.

See WordPress/gutenberg#45242.

Props ramonopoly, poena.
Fixes #58614.

 --Cette ligne, et les suivantes ci-dessous, seront ignorées--

M    trunk/src/wp-includes/class-wp-theme-json.php


git-svn-id: https://develop.svn.wordpress.org/trunk@56041 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme-json.php

@@ -313,10 +313,12 @@ class WP_Theme_JSON {
 	 * @since 5.8.0 As `ALLOWED_TOP_LEVEL_KEYS`.
 	 * @since 5.9.0 Renamed from `ALLOWED_TOP_LEVEL_KEYS` to `VALID_TOP_LEVEL_KEYS`,
 	 *              added the `customTemplates` and `templateParts` values.
+	 * @since 6.3.0 Added the `description` value.
 	 * @var string[]
 	 */
 	const VALID_TOP_LEVEL_KEYS = array(
 		'customTemplates',
+		'description',
 		'patterns',
 		'settings',
 		'styles',