Heading: Add border support

[aaronrobertshaw]

Part of: #43241

What?

Adopts border support for heading blocks.

Why?

• Offers greater design flexibility
• Improves consistency in design tooling with other blocks
• Block already has padding and background color support. This adds another missing piece.

How?

• Adopt all border supports.

Testing Instructions

1. In the site editor, add a Heading block to a page
2. Style all Heading blocks via Global Styles and confirm the editor and frontend display correctly
3. Override the global styles on a block instance and confirm they display appropriately in the editor and frontend

Screenshots or screencast

Screen.Recording.2024-07-15.at.12.29.39.PM.mp4

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Co-authored-by: aaronrobertshaw <[email protected]>
Co-authored-by: carolinan <[email protected]>

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