crouton extension and manifest v3

[dnschneid]

Got the notification that manifest v2 extensions might be disabled starting June 2024. I'd take that date with a grain of salt, but as the crouton extension currently relies on having a background DOM to do copy/paste sync, we'll need to figure out a way to migrate to manifest v3 or defeature clipboard sync entirely. Hopefully no other features (looks at xiwi) are impacted by the switch...

[CroutonIsFun]

Thanks for staying on top of this, David.