Service Workers can be tricky to work with in production, and often
it becomes necessary to deactivate an existing SW. This is trickier
than one might imagine - as long as clients on the old SW may exist
it is important to serve some script at the old SW URL. This commit
adds safety-worker.js to the published NPM package, which is useful
for that purpose. On install the SW unregisters itself which safely
and gradually allows older clients to update.
PR Close#21863
