Post Title: Add border block support

[akasunil]

What?

Add border block support to the Post Title block.

Part of #43247

Why?

Post Title block is missing border support.

How?

Adds the border block support in block.json

Testing Instructions

• Go to Global Styles setting ( under appearance > editor > styles > edit styles > blocks )
• Make sure that Post Title block's border is configurable via Global Styles
• Verify that Global Styles are applied correctly in the editor and frontend
• Edit template/page, Add Post Title block and Apply the border styles
• Verify that block styles take precedence over global styles
• Verify that block borders display correctly in both the editor and frontend

Please refer to the attached video for more information.

Screenshots or screencast

Blog-Home-.-Template-.-gutenberg-.-Editor-.-WordPress.14.webm

[github-actions]

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Co-authored-by: akasunil <sunil25393@git.wordpress.org>
Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>

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[aaronrobertshaw]

Thanks for the thorough PR description and test instructions 👍

✅ Global styles display correctly
✅ Block instance styles override global styles
✅ Styling is consistent between editor and frontend
✅ Box-sizing was already set to border-box for when parent enforces an explicit width

LGTM 🚢