Add workflow to insert PR preview links

[skylarkning]

Summary

Adds a GitHub Actions workflow that automatically inserts profiler preview links into PR descriptions when they are missing.

The workflow:

• Runs on pull_request_target
• Checks whether the PR body already has a deploy preview link
• Looks for profiler/share links in the PR body or linked issue body/comments
• Resolves share.firefox.dev links to the final profiler path
• Adds [Main] | [Deploy preview] links to the PR description

Testing

• node --test .github/scripts/add-preview-links.test.mjs
• node --check .github/scripts/add-preview-links.mjs
• node --check .github/scripts/add-preview-links.test.mjs
• Live-tested on my fork with a temporary issue and PR; the workflow successfully updated the PR body with the expected preview links.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.47%. Comparing base (de0953b) to head (ecfaf86).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6133   +/-   ##
=======================================
  Coverage   83.47%   83.47%           
=======================================
  Files         342      342           
  Lines       36097    36097           
  Branches    10098    10098           
=======================================
  Hits        30131    30131           
  Misses       5538     5538           
  Partials      428      428           

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