LibJS: Oops, "instanceof" was backwards!

Fix the "instanceof" operator to check if the constructor's prototype
property occurs anywhere in the prototype chain of the instance object.

This patch also adds Object.setPrototypeOf() to make it possible to
create a test for this bug.

Thanks to DexesTTP for pointing this out! :^)

Libraries/LibJS/Runtime/Object.cpp

@@ -44,6 +44,15 @@ Object::~Object()
 {
 }
 
+bool Object::has_prototype(const Object* prototype) const
+{
+    for (auto* object = m_prototype; object; object = object->prototype()) {
+        if (object == prototype)
+            return true;
+    }
+    return false;
+}
+
 Optional<Value> Object::get_own_property(const Object& this_object, const FlyString& property_name) const
 {
     auto value_here = m_properties.get(property_name);