[Cashtab Extension] Migrate to v3 manifest

Summary:
T2495

Migrate the Cashtab extension to v3 manifest

Chrome will (someday) discontinue support for the v2 manifest. In the meantime, they are taking a long time to review new versions with the v2 manifest.

I've updated the major version number as, going forward, manifest v3 code syntax rules will apply. Users of the extension do not have to perform any migration action.

Test Plan:
npm run extension, load the unpacked extension in chrome or brave (see README), and enable it. Disable the official cashtab extension if you have it.
Visit components.cashtab.com
Confirm that the "get address" component and the "Cashtab Badge" components update the extension
Try it with Paybutton, e.g. https://proofofwriting.com/181/, confirm extension opens

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14692