Breadcrumbs: Add setting to show trailing separator

[himanshupathak95]

What?

Closes #78926

This PR introduces a new block attribute, showTrailingSeparator, for the Breadcrumbs block. When enabled, it applies a has-trailing-separator CSS class to the block wrapper, forcing the CSS ::after separator to render on the :last-child item.

Why?

This resolves an issue where users hiding the "current item" could not append a trailing separator to the end of the remaining breadcrumb trail.

Testing Instructions

1. Insert a Breadcrumbs block into a post.
2. Under the block settings, toggle ON the new Show trailing separator option.
3. Verify that a trailing separator appears at the end of the breadcrumb trail.
4. Toggle OFF the Show current breadcrumb option.
5. Verify that the current item disappears, but the trailing separator remains correctly attached to the preceding item.
6. Publish the post and verify the changes reflect correctly on the frontend.

Screenshots or screencast

Screen.Recording.2026-06-04.at.20.45.36.mov

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Co-authored-by: himanshupathak95 <abcd95@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: ntsekouras <ntsekouras@git.wordpress.org>
Co-authored-by: mrwweb <mrwweb@git.wordpress.org>

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