Feat: Add “Show related posts” toggle & "same term" filter/flag to display related posts.

[sarthaknagoshe2002]

What?

Closes #77099

Adds a “Show related posts” toggle to display posts sharing the same categories/tags as the current post.

Why?

This is a longstanding gap for theme authors. The most common use case is a “Related Posts” section at the bottom of a single post. Today, the Query Loop block requires selecting explicit terms and has no awareness of the current post’s terms.

How?

Added a toggle labeled “Show related posts” for same-term filtering.
Uses core-data to fetch the current post’s categories/tags dynamically.
Automatically update the block query when terms change.

Testing Instructions

1. Edit a post with multiple categories and tags.
2. Insert the Query loop block.
3. Toggle “Show related posts” on, the block should list posts with the same categories/tags.
4. Toggle “Show related posts” off, manual include/exclude lists should apply instead.
5. Save the post, confirm the block list remains correct in the editor and on the frontend.

The Theme authors can use __experimentalSameTerm flag to enable this feature.

Screenshots or screencast

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Co-authored-by: sarthaknagoshe2002 <sarthaknagoshe2002@git.wordpress.org>
Co-authored-by: henriqueiamarino <iamarinoh@git.wordpress.org>

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