Make version that works with Manifest v3

[imurray]

Google are going to force Manifest v2 extensions like this one to migrate to v3 at some point (maybe in 2022?). Without updating how the extension works, it won't work in Chrome at that point. (The extension will continue to work in Firefox for the foreseeable future.)

From a quick look, it looks like it should be possible to make a manifest v3 version of the extension, and it won't be that hard. However, as I don't actually use Chrome myself, I haven't quite found the motivation to do it yet.

[imurray]

I've created a manifest3 branch that seems to work in Chrome (Chromium 89) with Manifest 3.

I'm in no immediate rush to publish this version in the Chrome Web Store, I anticipate the current version will keep working for at least a year. I'll wait until Manifest V3 is a bit older. I also intend to create a unified code base that builds Firefox and Chrome extensions.

[imurray]

After many delays, Google are now starting to slowly disable mv2 extensions, so I published a version of redirectify based on the manifest3 branch.

Frustratingly, the permissions still need to be broad and scary sounding, even though the extension has no code, and a static set of redirects.

The manifest v3 version doesn't work in Firefox at the moment. I'm going to wait and see what changes Firefox makes before investing any effort to reunify the branches for Firefox and Chrome, and/or add any features.