add shell.nix with npm serve

.envrc

@@ -0,0 +1 @@
+use nix;

shell.nix

@@ -0,0 +1,5 @@
+{ pkgs ? import <nixpkgs> {} }: pkgs.mkShell {
+  buildInputs = with pkgs; [
+    nodePackages.serve
+  ];
+}