Add Threads embed block variation

[pestevez]

What?

Adds Threads as an embed block variation, allowing users to embed Threads posts directly from the block inserter.

Note: This PR depends on Threads being registered as an oEmbed provider in WordPress Core. Threads pages do not include oEmbed discovery <link> tags, so the embed will only resolve once the Core change lands. See: Core Trac ticket / PR.

Why?

Threads recently updated their oEmbed API to drop the access token requirement, making tokenless embedding possible.

This brings Threads in line with other social platforms that already have embed block variations.

How?

• Added embedThreadsIcon to icons.js using the same SVG path already used by the social-link Threads icon
• Added a Threads variation to variations.js with URL patterns supporting both threads.com and threads.net domains
• Supported URL formats:
  • https://www.threads.com/@{username}/post/{shortcode}
  • https://www.threads.com/t/{shortcode}

Testing Instructions

1. Ensure Threads is registered as an oEmbed provider in Core (or apply the patch from [Core PR link])
2. Create a new post
3. Open the block inserter (/) and search for "Threads"
4. The Threads embed block should appear with its icon
5. Add the block and paste a Threads URL (e.g. https://www.threads.com/@zuck/post/C1234567890)
6. Verify the embed resolves and renders

Testing Instructions for Keyboard

1. Press / to open the block inserter
2. Type "Threads" and press Enter to select the Threads embed block
3. Paste a Threads URL into the URL field and press Enter
4. Verify the embed preview loads

Screenshots or screencast

Before	After
No Threads option in embed blocks

Use of AI Tools

Created using Claude Code (Opus 4.6).

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