feat(langs,nix): Add nix language support

[Fryuni]

Uses rnix-lsp as language-server for Nix files.

For Nix users the rnix-lsp is available on nixpkgs.

Enabling the linter feature enable the following:

• Code Actions and diagnostics using statix
• Diagnostics using deadnix for dead code
• Formatting using nixpkgs-fmt

All of those binaries are also available on nixpkgs and can be individaually disables (or switched in the case of nixpkgs-fmt)

[connorgmeehan]

Looks great, thanks for the PR!