Update to support Manifest V3

[alikoneko]

It seems that Chrome is going through with taking down stuff that uses the old Manifest V2, so it'd be nice if this extension got updated to support Manifest V3 so that this extension doesn't get taken down.

[redac]

👋🏼 @sad @69 I think I've tackled this in PR #21. Feel free to merge if it looks good to you!