Migrate to manifest v3

[infokiller]

Manifest v3 landed in Chromium stable 88 (released on January 2021). It's unclear when v2 will stop working.
We need to be careful about Firefox having support: https://blog.mozilla.org/addons/2021/05/27/manifest-v3-update/

[infokiller]

Deadline seems to be January 2023: https://developer.chrome.com/docs/extensions/mv3/mv2-sunset/

I'm still not sure about the support status in Firefox.

[infokiller]

As of today (2022-05-09), Firefox still doesn't seem to have (good?) support for MV3. I played with using symlinks to create two different source directories, one for Firefox and one for Chrome, which use symlinks to the source files, but it failed and it seems like an issue with Chrome.
This means that as long as Firefox doesn't support MV3, we won't be able to support loading "from source" for both Chrome and Firefox, and we'll have to use a build step.

[infokiller]

It seems we're also blocked on fregante/webext-dynamic-content-scripts#18

[infokiller]

According to https://developer.chrome.com/docs/extensions/mv3/mv2-sunset/ the original deadline of Jan 2023 (for disabling MV2 in non-stable channels and removing the Featured Badge) is pushed to an unknown date, probably March 2023 or later.