feat: add nix flake

[Saghen]

Adds a nix flake so that nix users may use the language server in their config like so:

inputs = {
  emmylua-analyzer = {
    url = "github:cppcxy/emmylua-analyzer-rust";
    inputs.nixpkgs.follows = "nixpkgs";
  };
};
...
environment.systemPackages = [ inputs.emmylua-analyzer.packages.${pkgs.system}.default ]