[CI] Fix submodule sync & pr-action adjustments

[chalin]

• NPM script changes:
  • Creates fix:all, as the canonical name for fix, which is kept as an alias
  • Creates fix:submodules, which first gets all submodules and then syncs them (previously, just syncing without first updating the submodules could fail)
  • Creates fix:refcache to match PR-action command name
  • Renames sync to _sync, since most users will not use the more robust fix:submodules
• Adjusted the pr-action workflow to invoke npm run fix:<cmd> for the various commands. Refactored the case statement to make it more DRY
• Minor prose tweaks and script-name adjustments to Contributing page