Add nixd, new lsp server for Nix

[arthsmn]

I'm not setting it as default, but I think it can be discussed, as the default one is abandoned and this one is much more powerfull than nil.

Edit: found out that rnix-lsp is not available in nixos-unstable or nixos-24.05 (latest stable), so it is basically not supported in NixOS anymore.

[manateelazycat]

Thanks