Manifest v3 compatibility

[jelmervdl]

Use manifest v3 for Firefox, and Chrome.

In Chrome, we use chrome.offscreen to have access to Worker. Unfortunately, that means yet another layer of indirection: Translations now go from the page -> content-script -> background-script -> offscreen script -> worker -> wasm runtime 😭

Todo:

• Restore tab state when the service worker is unloaded
• Communicate translation cancellation to worker in the Offscreen page: problem is that AbortSignal doesn't make it across the message passing boundary.

What I'd really really like:

• Tests! I can't keep testing two browsers manually :(

[jelmervdl]

For some reason you need to enable the access to statmt and translatelocally explicitly. Quite annoying. I wonder if that's something missing in the installation flow because I'm running in developer mode or not.