Add shell.nix including elan and stack

The idea is that `elan` should be used to install `lean` rather than directly using Nix, because of issues with packaging `mathlib` such as discussed here: leanprover/lean4#5122.

shell.nix

@@ -0,0 +1,7 @@
+{ pkgs ? import <nixpkgs> {} }:
+pkgs.mkShell {
+  packages = with pkgs; [ 
+    elan
+    stack
+  ];
+}