infra: add shell.nix

The patch introduces a shell.nix that helps creating a developer
environment for running tests:

$ nix-shell

Author: ligurio

shell.nix

@@ -0,0 +1,32 @@
+# Nix development shell for lunapark
+#
+# Usage:
+#   nix-shell              # Enter development environment.
+#   nix-shell --pure       # Enter pure (isolated) environment.
+#
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  name = "lunapark-dev";
+
+  buildInputs = with pkgs; [
+    cbmc
+    cbmc-viewer
+    clang
+    cmake
+    git
+    gnumake
+    libunwind
+    ninja
+    protobuf_21
+    readline
+    xz
+    zlib
+  ];
+
+  shellHook = ''
+    echo "lunapark Development Environment"
+    echo "  cmake --workflow --preset luajit"
+    echo "  cmake --workflow --preset lua"
+  '';
+}