Add Thunderbird

[d-dervishi]

Description

This adds options to configure Thunderbird as an email client based on the accounts.email.accounts options.

I've only been able to test the module on NixOS (22.05), but I've also adapted the paths for Darwin based on the Thunderbird docs. I would be happy if someone could help me test that on other platforms :)

Checklist

• [x] Change is backwards compatible.

• [x] Code formatted with ./format.

• [x] Code tested through nix-shell --pure tests -A run.all.

• [x] Test cases updated/added. See example.

• [x] Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • [x] Added myself as module maintainer. See example.

  • [x] Added myself and the module files to .github/CODEOWNERS.