Social Link block: Add automatic RSS feed link option

[dhruvikpatel18]

Closes #52202

What?

This PR adds a checkbox option "Link to the RSS feed of this site" when adding an RSS social icon, which automatically appends "/feed/" to the URL.

Why?

When users add an RSS icon to their Social Icons block, they often need to manually enter the default WordPress feed URL ("/feed/"). Since this is the standard WordPress RSS feed path, providing an automatic option improves the user experience.

Testing Instructions

1. Add a Social Icons block to a post or page
2. Add an RSS icon to the Social Icons block
3. Click on the RSS icon to open the URL popover
4. Notice the new checkbox "Link to the RSS feed of this site"
5. Enter a URL (e.g., "https://example.com/")
6. Check the checkbox and observe that "/feed/" is appended to the URL
7. Uncheck the checkbox and observe that "/feed/" is removed from the URL
8. Enter a URL that ends with "/feed/" manually and observe that the checkbox is automatically checked

Screenshots or screencast

RSS.feed.url.mov

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Co-authored-by: dhruvikpatel18 <dhruvik18@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: hanneslsm <hanneslsm@git.wordpress.org>

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[Mamaduka]

Thanks for contributing, @dhruvikpatel18!

Closing this as "maybelater"; please see the issue for the related discussion.