Comments Pagination: Add example attribute to block

[Infinite-Null]

Part of #64707

What?

This PR adds an example attribute to the Comments Pagination block’s block.json to provide a default paginationArrow.

Testing Instructions

1. Open a post or page in the WordPress block editor.
2. Open the block inserter by clicking the “+” button.
3. Add comments block and search for comments pagination block.
4. Hover over the comments pagination block in the inserter.

Screenshots

Before	After

[github-actions]

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

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[carolinan]

I found it confusing that the comments pagination and comment pagination numbers share the same preview, but I think it is better than leaving the empty grey box.

[carolinan]

Screenshot of Appearance > Editor > Styles > Blocks > Comments pagination > Stylebook