Added a nix-shell template and a rust dev shell

nix-shells/template.nix

@@ -0,0 +1,21 @@
+# https://nix.dev/tutorials/declarative-and-reproducible-developer-environments
+let
+  # nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-23.11";
+  nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/master";
+  pkgs = import nixpkgs { config = {}; overlays = []; };
+in
+
+pkgs.mkShellNoCC {
+  packages = with pkgs; [
+    cowsay
+    lolcat
+  ];
+
+  # Environment Variables
+  GREETING = "Hello, Nix!";
+
+  # Startup command
+  shellHook = ''
+    echo $GREETING | cowsay | lolcat
+  '';
+}