Automatically assign plugin-specific milestones on pull requests.

[dilipom13]

Summary

Fixes #2541

Implements automatic assignment of a plugin-specific milestone when a pull request is opened or updated. The workflow counts changed lines under each plugins/<slug>/ path, picks the plugin with a unique highest total, and sets the milestone (preferring <slug> n.e.x.t when it exists). Related to the existing plugin label automation in .github/workflows/add-pr-labels.yml.

Note: PR #2586 addresses the same issue; this branch was prepared locally for testing and to offer help with review or follow-up fixes if maintainers prefer.

Relevant technical choices

• Milestone assignment runs in the same pull_request_target workflow as plugin labels, using actions/github-script and shared logic in bin/plugin/lib/assign-plugin-milestone.js.
• Pull request files are fetched via the GitHub API (no checkout of the contributor branch for assignment logic).
• Cross-plugin renames count additions toward the destination plugin and deletions toward the source plugin.
• If the PR already has a milestone for the selected plugin, it is left unchanged.
• Ties or ambiguous open milestones leave the milestone unchanged and log an informational message.
• Adds npm run test-unit-js (node --test) and runs it in the JS lint workflow.
• The optional changelog-label enhancement from Add workflow to automatically assign milestone #2541 is out of scope for this change.

Use of AI Tools

No AI tools were used to author the code changes. AI was not used to write the implementation.

[github-actions]

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Co-authored-by: dilipom13 <dilip2615@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.35%. Comparing base (0a51c32) to head (4b78e82).

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #2593   +/-   ##
=======================================
  Coverage   70.35%   70.35%           
=======================================
  Files          91       91           
  Lines        7867     7867           
=======================================
  Hits         5535     5535           
  Misses       2332     2332           

Flag	Coverage Δ
multisite	70.35% <ø> (ø)
single	35.17% <ø> (ø)

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[mukeshpanchal27]

What is reasons behind this new script?

[dilipom13]

What is reasons behind this new script?

Hello @mukeshpanchal27, good question thanks for asking.

This PR adds bin/plugin/lib/assign-plugin-milestone.js with unit tests in assign-plugin-milestone.test.js.
Those tests use Node’s built-in test runner (node:test), not PHPUnit or Playwright.

The new test-unit-js script in package.json is a small wrapper so contributors and CI can run them the same way:

npm run test-unit-js