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.

git-svn-id: https://develop.svn.wordpress.org/trunk@55176 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.json

@@ -325,6 +325,20 @@
 				}
 			}
 		},
+		"shadow": {
+			"presets": [
+				{
+					"name": "Natural",
+					"slug": "natural",
+					"shadow": "0 .2rem .3rem 0 rgba(0,0,0, 0.3), 0 .5rem .6rem 0 rgba(0,0,0, 0.4)"
+				},
+				{
+					"name": "Sharp",
+					"slug": "sharp",
+					"shadow": ".5rem .5rem 0 0 rgba(0,0,0, 0.4)"
+				}
+			]
+		},
 		"spacing": {
 			"blockGap": null,
 			"margin": false,