Add nix package

[Myzel394]

Libreddit is available on nixos (though not in full). Would be nice if redlib is available as a nix package too with proper configurable settings.

[sigaloid]

~~Fixed by https://github.com/redlib-org/redlib/pull/128!~~

Oops, not a package but a flake. But I cannot maintain this as a non-nix user so this would be something for another contributor

[jervw]

Since last week, the Libreddit NixOS service has been dropped and renamed to Redlib in NixOS/nixpkgs#327939. The old package has also been removed.

Options, although limited, can be found here

[jervw]

Therefore, I believe this issue can be closed as resolved.