Feat: Add theme.json support for textShadow

[USERSATOSHI]

What?

Part of #47904
Related with #71419

This PR adds theme.json support for the textShadow typography field.

It updates the schema, theme.json defaults, PHP theme.json processing, and the style engine so settings.typography.textShadow and styles.typography.textShadow are recognized and can generate text-shadow CSS.

Why?

The larger textShadow effort was split into smaller steps.

The first step is making theme.json understand the field end to end. Without this, themes cannot declare textShadow in theme.json, and Gutenberg cannot consistently validate and generate styles for it.

Landing the theme.json layer first reduces scope and gives us a stable base for the later UI work.

How?

This PR adds textShadow support in the theme.json pipeline by updating:

• the theme.json schema to allow the setting and style field
• default theme settings to expose settings.typography.textShadow
• WP_Theme_JSON_Gutenberg metadata and allowlists so the field is accepted and mapped to the text-shadow CSS property
• the style engine typography definitions so JS style generation includes textShadow
• documentation and focused tests for the new field

Testing Instructions

1. Start the local Gutenberg environment.
2. Create a Test Theme.
3. Add this theme.json:

{
	"$schema": "../../schemas/json/theme.json",
	"version": 3,
	"settings": {
		"appearanceTools": true,
		"layout": {
			"contentSize": "840px",
			"wideSize": "1100px"
		},
		"typography": {
			"textShadow": true
		}
	},
	"styles": {
		"elements": {
			"heading": {
				"typography": {
					"textShadow": "2px 2px 4px rgba(0,0,0,0.35)"
				}
			}
		},
		"blocks": {
			"core/paragraph": {
				"typography": {
					"textShadow": "1px 1px 0 #ff0000"
				}
			}
		}
	}
}

4. Open a post or page.
5. Insert a Heading block and a Paragraph block.
6. Confirm the styles are applied in the editor.
7. View the post on the frontend and confirm the styles are applied there as well.
8. Inspect the rendered CSS and verify that text-shadow is generated from the theme.json values.

Screenshots or screencast

Use of AI Tools

Github Copilot used to generate PR description and test examples.

[USERSATOSHI]

Duplicate of #73320