Add branch targeting check to analyze-pr workflow (#55383)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55383

Adds a branch targeting validation that replaces the equivalent Danger.js check.
  The check validates that PRs target either `main` or a `-stable` branch, and
  automatically adds the "Pick Request" label when targeting stable branches.

  - Created `checkBranchTarget.js` - pure function returning validation message
  - Added check step to `annotate-pr.yml` that calls the JS and adds labels
  - Added `issues: write` permission for label functionality

Changelog: [Internal]

Reviewed By: huntie

Differential Revision: D91685081

fbshipit-source-id: 87e7124f7d825b51cb791dc94720c487ff95d414