added shell.nix

shell.nix

@@ -0,0 +1,21 @@
+ let
+   nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-24.05";
+   pkgs = import nixpkgs { config = {}; overlays = []; };
+ in
+
+ pkgs.mkShellNoCC {
+   packages = with pkgs; [
+     cowsay
+     lolcat
+     neovim
+     fish
+     git
+     tmux
+   ];
+
+   GREETING = "Hello, Nix!";
+
+   shellHook = ''
+     fish
+   '';
+ }