Move to Manifest V3

[kumar-ish]

Chrome extensions have been heavily recommended / are being required to move to Manifest V3 (see: https://developer.chrome.com/docs/extensions/mv3/intro/).

AFAICT, to work on this extension locally requires it to be running Manifest V3 right now.

Similar issue on rikaikun (although outdated since the timelines are now actually approaching / Chrome is pushing it to be done ASAP)

[kumar-ish]

There's a few features I would like to have in the extension (particularly #31 and #45), and I would like to work on them. I can investigate and see if I have the capacity / capability w.r.t. experience or what-have-you to work on it and let you know -- it is a very substantial refactor so it'll be a big job re-writing and reviewing.