Add support for more browsers

[M-J-Robbins]

Build a version that will work in

• Firefox
• Edge
• Opera
• Safari

This may also work as a thunderbird extension and potentially more email clients.

The above updates either mean adding a version built with manifest V2 or waiting for those browsers to move to V3. Firefox predict support by end of 2022

[M-J-Robbins]

I've just updated my version of Edge and Opera and it's working there already