Add shell.nix for easier testing on NixOS

shell.nix

@@ -0,0 +1,21 @@
+{
+  pkgs ? import <nixpkgs> { },
+}:
+
+pkgs.mkShell {
+  buildInputs = [
+    pkgs.python312
+    pkgs.python312Packages.virtualenv
+  ];
+
+  shellHook = ''
+    if [ ! -d .venv ]; then
+      virtualenv .venv
+      . .venv/bin/activate
+      pip install -r requirements.txt
+      pip install -e .
+    else
+      . .venv/bin/activate
+    fi
+  '';
+}