Create Thunderbird overlay

[Finchiedev]

Hello! I noticed Nix didn't seem to have Thunderbird Daily packaged anywhere, so thought this overlay might be a good place for it to live. The PR itself is very simple, just copying firefox-overlay.nix into a new file thunderbird-overlay.nix and tweaking the configurations to make everything work. I'm not very comfortable with Nix but it feels like there could be a better way to do this, very happy to make any changes to improve code quality.

Apologies if I'm contributing to the wrong place, please feel free to close this PR if so!