Migrate Extension to MV3

[Jyyjy]

Copied from apache/flagon-useralejs#421

This guide offers some best practices on building a cross browser extension. Including an example of how to remedy the differences of service workers and background scripts, which has been what is holding back MV3.

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Build_a_cross_browser_extension

Edit: I only plan on targeting Chrome and Firefox. But I encourage others to add support for their favorite browsers.