♻️ Refactored for manifest v3 support

- moved to service worker for Chrome
- polyfill removed as we didn't need it
- instead added suggested workaround from https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/background#browser_support
- manifest set to version 3

All in all this is once more a shitty situation: Chrome now requires Manifest v3 and only supports `service_worker` scripts for that, whereas Firefox doesn't support `service_worker` scripts yet and still depends on `background` scripts. Thankfully Chrome no longer seems to refuse extensions that contain a `background` script, it just ignores it. And Firefox ignores `service_worker`. Safari apparently supports both but prefers `service_worker`. In any case, I've rewritten the background script here to work as both and in both Chrome (`chrome.*`) and other browsers (`browser.*`). In other news, fsck Google, Chrome really is the new Internet Explorer...

Author: foosel