Manifest version 3

[stefansundin]

The clock is ticking on updating extensions to MV3, so I decided to take a stab at it this weekend. I ended up rewriting the extension and it is now much cleaner than before. I actually rewrote it multiple times since I always found small issues and minor ways to do things better.

As as nice side effect, I realized that I can duplicate tabs and then change which tabs are active, so the extension no longer needs the tabs permission for the Open new tabs in the background feature. 🎉

I pushed the changes to the repository but I haven't submitted it to the store yet. I plan on testing it for a week or so before I do that. If anyone else decides to test it out then please let me know if you find any bugs or issues. Thanks!