Heading: Add missing typography support

[aaronrobertshaw]

Related:

• Tracking: Addressing Design Tooling Consistency #43241
• Tracking: Typography Design Tools Consistency #43242

What?

Adds missing typography support to the Heading block.

Why?

• Improves consistency of our design tools across blocks.

How?

• Opts into missing text-decoration typography support.

Testing Instructions

1. Edit a post, add a Heading block, and select it
2. Check that the text-decoration control is now available
3. Ensure text decoration is applied both in the editor and frontend
4. Test styling via theme.json (Note: Text decoration isn't available in Global Styles)

Example theme.json snippet:

			"core/heading": {
				"typography": {
					"textDecoration": "underline"
				}
			},

Screenshots or screencast

[glendaviesnz]

Worked for me:
✅ In editor and frontend
✅ When added to theme.json showed in editor and frontend