mkFirefox: add policy to accept added extensions

[Mikilio]

This commit adds extensions added via
profiles..extensions.packages to the policy. In consequence, the extensions are added without complaint and don't need to be enabled manually.

Description

While switching browser, I noticed that the current method of installing extensions, does not enable them. This is because the only firefox is allowed to install extensions, and it only provides one way to do so automatically.
As such, I have improved the method of installing extensions by using the intended way by mozilla to do this, without sacrificing reproducibility. This is achieved by "downloading" extensions from the nix store.

NOTE:

• The extensionPath uuid constant became obsolete through this because extensions don't need to be bundled anymore.
  This involves @rycee to change his convenient output derivations for firefox modules.
• This change relies on the fact that firefox modules from @rycee NUR repository expose a addonId. This needs to be documented somewhere
• nix fmt . made some unnecessary changes.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example. (not applicable)

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  Maintainers and Involved:
  @chayleaf
  @onny
  @brckd
  @kira-bruneau

[JuneStepp]

FYI, I've tried this before and ran into some issues. I can't remember for sure, but it may have been to do with extension updates. Just take this as a reminder to test well.

[Mikilio]

I left this as a draft for now, as I'm just gathering input. I will run this myself for a while to see if there are any issues, and I'll keep an extra eye on extension updates (though in theory it should work, as the installation_url will change).