Added nix shell

Author: bwbush

analysis/markov/shell.nix

@@ -0,0 +1,12 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    lean4
+    elan
+    vscode
+  ];
+  shellHook = ''
+    echo "Entering Nix shell with Lean 4, Elan, and VS Code..."
+  '';
+}