Shadow: add shadow presets support via theme.json

[madhusudhand]

What?

This adds support for default and theme shadow presets via theme.json

It ads following default presets to Gutenberg bundled theme.json

• Natural
• Sharp

Testing Instructions

1. This change should create two default preset variables --wp--preset--shadow--natural and --wp--preset--shadow--sharp in global style sheet.
2. Add the following styles in the current active theme.

"settings": {
		"shadow": {
			"palette": [
				{
					"name": "Natural",
					"slug": "theme-natural",
					"shadow": "5px 5px 0px -2px #FFFFFF, 5px 5px #000000"
				},
				{
					"name": "Crisp",
					"slug": "theme-crisp",
					"shadow": "5px 5px #000000"
				},
				{
					"name": "Sharp",
					"slug": "theme-sharp",
					"shadow": "5px 5px 0 0 #999999"
				},
				{
					"name": "Soft",
					"slug": "theme-soft",
					"shadow": "5px 5px 10px 0 #999999"
				}
			]
		}
}

3. Verify that it creates the respective preset variables in global style sheet.
4. Apply shadow to any block such as button in the current active theme.

{
    "styles": {
         "elements": {
              "button": {
                  "shadow": "var(--wp--preset--shadow--natural)"
              }
         }
    }
}

5. Verify the button shadow.

This is a PR to add support via theme.json.
Verify end to end feature including UI in global styles from here #46502

Related issues

Partial Fix for #44651

[oandregal]

Thanks for splitting the work in smaller steps!

I've tested a variety of scenarios: defining the presets in the theme.json of the theme instead of using the theme.json bundled by Gutenberg, using the shadow in a block. I've also tested that style variations work the same as the main theme.json of the theme (needed to merge #46810 into this one). This is ready to ship.