add devShell to flake template snippet

snippets/nix.snippets

@@ -33,7 +33,7 @@ snippet flake
         let
           pkgs = nixpkgs.legacyPackages.\${system};
         in
-        {
+        rec {
           packages = rec {
             $1 = pkgs.stdenv.mkDerivation {
               name = "$1";
@@ -43,6 +43,12 @@ snippet flake
             };
             default = $1;
           };
+          devShells.default = {
+            inputsFrom = [ packages.default ];
+            packages = with pkgs; [
+              $3
+            ];
+          };
         }
       );
   }