feat(ensure): create ensure module

This module serves to ensure specific directories exist, however in the
future it could be used for other things that need to be 'ensured'.

modules/modules/global.nix

@@ -18,4 +18,7 @@ with lib.hlissner;
 
   # Allow non-root users to allow other users to access mount point
   programs.fuse.userAllowOther = mkDefault true;
+
+  # Ensure certain necessary directories always exist
+  services.ensure.directories = [ "/mnt" ];
 }