Manifest V3 upgrade

[tjhorner]

Over the past few years, Google has repeatedly threatened to deprecate Manifest V2 but then either temporarily suspended or extended the deadline every single time. But they will do it eventually...

If we want to keep the extension on the Chrome web store (and likely other Chromium-like stores) then we will need to move to Manifest V3. This issue is for discussing and tracking what is required for the upgrade.

Google suggests we complete this before June 2024:

We will begin disabling Manifest V2 extensions in pre-stable versions of Chrome (Dev, Canary, and Beta) as early as June 2024, in Chrome 127 and later. [...] For extensions publishers who still publish Manifest V2 extensions, we highly recommend completing migration to Manifest V3 before June 2024.

And they have published a migration guide