Attempt at adding Manifest V3 support to Compose Key

[gutschke]

I made an attempt to convert the compose key extension to Manifest V3. But I am very unclear on how the transition to this new API is supposed to work. There are a bunch of API changes for the background service worker, that I addressed as best as I could. Please review before blindly applying these changes.

In particular, the service worker no longer has access to local storage and has to use a Chrome-specific API. I don't know if I have done this correctly, or if my changes interfere with sync'ing of settings across browsers. I don't quite understand the interactions between "chrome.storage.local" and "chrome.storage.sync".

Furthermore, it looks as if service workers don't have access to "KeyboardEvent". That affects a bunch of code that handles composed sequences. But I believe it really only matters for the location of the key on the keyboard (i.e. number pad vs. main area). And maybe, it's OK to skip that? That's why my changes do. But maybe there is a way to retain this data by refactoring the code? manifest-v3.diff.txt