Add shell.nix

That way I can use nix-shell to execute the wrapper.
nix will download all runtime dependencies and adjust path within nix
shell.

shell.nix

@@ -0,0 +1,14 @@
+{ pkgs ? import <nixpkgs> { } }:
+
+pkgs.mkShell {
+  name = "Videocutting";
+  buildInputs = [
+    pkgs.ffmpeg
+    pkgs.php82
+    pkgs.php82Packages.composer
+  ];
+
+  shellHook = ''
+    export PATH="$(pwd)/:$PATH"
+  '';
+}