Upgrade to web manifest v3

From briefly looking at the retooling branch code, here are the known changes required to support manifest v3. It looks like the majority of this work is related to moving to a service worker.

[ ] Move background DOM manipulation to a offscreen document
[ ] Move background actions to service worker
[ ] Switch off local storage API (#89)
[ ] Replacing some unsupported APIs like tabs.sendMessage()

There may be additional issues migrating to v3. Cross-browser compatibility will also take some careful testing considering Firefox and Safari have different implementations than Chrome.

keithjgrant / omnibear

Upgrade to web manifest v3 #107