Post Comments Form: Add Border Block Support

[shail-mehta]

What?

Add border block support to the Post Comment Form block.

Part of #43247

Why?

Post Comment Form block is missing border support.

How?

Adds the border block support in block.json.

Testing Instructions

• Go to Global Styles Settings ( Under Appearance > Editor > Styles > Edit styles > Blocks ).
• Make sure that Post Comment Form block's border is Configurable via Global Styles.
• Edit template/page, Add Post Comment Form block and Apply the border Styles.
• Verify that Post Comment Form block styles take precedence over global Styles.
• Verify that Post Comment Form block borders display correctly in both the Editor and Frontend.

Screenshots or Screencast

add-border-support-in-post-comment-forms.mp4

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: shail-mehta <shailu25@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[t-hamano]

LGTM!

• ✅Global styles are applied correctly.
• ✅Styles are consistent between the editor and the front end.
• ✅Can override styles from block instances.
• ✅box-sizing is already applied.

One thing I noticed is that the corners of the border are sometimes hidden by the placeholder when it's visible, i.e. when comments are disabled. However, this also happens with other blocks, so I don't think it's necessarily a bug that needs to be fixed, and I think it can be addressed in a follow-up if necessary.