Border support added to comments

[rinkalpagdar]

What?

Add border block support to the Comments block.
Part of #43247

Why?

Comments block is missing border support.

How?

Adds the border block support in block.json

Testing Instructions

Testing Instructions for Keyboard

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

Screenshots or screencast

[github-actions]

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[github-actions]

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[michalczaplinski]

Thank you for the PR @rinkalpagdar ! 🙏

This tests well for me both on the frontend and in the editor.