Themes: Add shadow presets support for theme.json.

Adds shadow presets support for `theme.json` within the `WP_Theme_JSON` by 

* Adding it to presets metadata and valid settings.
* Defining the default shadow presets in Core's `theme.json`.

Includes PHPUnit tests. 

References:
* [https://github.com/WordPress/gutenberg/pull/46813 Gutenberg PR 46813]
* [https://github.com/WordPress/gutenberg/pull/47272 Gutenberg PR 47272]

Follow-up to [54162], [52049], [50973].

Props madhudollu, mamaduka, oandregal, scruffian, hellofromTonya.
Fixes #57559.
Built from https://develop.svn.wordpress.org/trunk@55176


git-svn-id: http://core.svn.wordpress.org/trunk@54709 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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

@@ -115,6 +115,7 @@ class WP_Theme_JSON {
 	 *              `use_default_names` preset key, and simplified the metadata structure.
 	 * @since 6.0.0 Replaced `override` with `prevent_override` and updated the
 	 *              `prevent_override` value for `color.duotone` to use `color.defaultDuotone`.
+	 * @since 6.2.0 Added 'shadow' presets.
 	 * @var array
 	 */
 	const PRESETS_METADATA = array(
@@ -176,6 +177,15 @@ class WP_Theme_JSON {
 			'classes'           => array(),
 			'properties'        => array( 'padding', 'margin' ),
 		),
+		array(
+			'path'              => array( 'shadow', 'presets' ),
+			'prevent_override'  => array( 'shadow', 'defaultPresets' ),
+			'use_default_names' => false,
+			'value_key'         => 'shadow',
+			'css_vars'          => '--wp--preset--shadow--$slug',
+			'classes'           => array(),
+			'properties'        => array( 'box-shadow' ),
+		),
 	);
 
 	/**
@@ -294,7 +304,7 @@ class WP_Theme_JSON {
 	 *              and `typography`, and renamed others according to the new schema.
 	 * @since 6.0.0 Added `color.defaultDuotone`.
 	 * @since 6.1.0 Added `layout.definitions` and `useRootPaddingAwareAlignments`.
-	 * @since 6.2.0 Added `dimensions.minHeight`.
+	 * @since 6.2.0 Added `dimensions.minHeight`, 'shadow.presets', and 'shadow.defaultPresets'.
 	 * @var array
 	 */
 	const VALID_SETTINGS = array(
@@ -338,6 +348,10 @@ class WP_Theme_JSON {
 			'padding'           => null,
 			'units'             => null,
 		),
+		'shadow'                        => array(
+			'presets'        => null,
+			'defaultPresets' => null,
+		),
 		'typography'                    => array(
 			'fluid'          => null,
 			'customFontSize' => null,