Motivation for this change

Spin off from #103156

This offers an alternative FHS based packaging for the tor browser. This may be useful for some people (due to faster updates, or perhaps less chance of breakage, due to being closer to upstream), but obviously has its drawbacks. It is not as close to the spirit of Nix / NixOS and will waste disk space, by keeping a separate copy of the (compressed) source in the store, in addition to the one in the local prefix.
Another rationale for writing this may be that mozilla is known for increasingly trying to lock down the browser, thereby sadly making the kind of packaging as required in nixpkgs more difficult to do properly.

Implementation details

• It uses .local/share/tor-browser-mutable/ as prefix (separate from the prefix of tor-browser-bundle-bin), so they should not collide.
• It adds some things to the passthru attribute of tor-browser-bundle-bin, to keep versions in sync; hope that's a valid thing to do.

Things done

• running works and NoScript / addons as well
• checked that the tor browsers self update mechanism works

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.