Migrate Chrome extension to Manifest V3

[Pharb]

Notes:

• "suggested_key" auto-conversion of "Ctrl" to "Command" on Mac does not work as documented by MDN / Chromium (I suspect this is a bug in Chromium, workaround is simple enough)
• service worker does not follow current best practises regarding storage and timers (good enough for now, can be further improved)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (8e86e53) to head (e34cb50).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #261 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 9 9 Lines 738 740 +2 Branches 135 137 +2 ========================================= + Hits 738 740 +2 ```

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