[feature request] add new `isBaseObject` predicate function

[DerekNonGeneric]

This predicate function seems like it would be useful.

private static boolean isBaseObject(Scriptable s) {
  // A direct instance of Object is the only Object whose prototype's
  // prototype is null.
  Scriptable proto = s.getPrototype();
  return proto != null && proto.getPrototype() == null;
}

Refs: http://cajadores.com/docs/java/com/google/caja/util/RhinoAsserts.html#isBaseObject(org.mozilla.javascript.Scriptable)