LibWeb: Use IterationDecision in single level Node iteration methods

`Node::for_each_child()` and `Node::for_each_child_of_type()` callbacks
now return an `IterationDecision`, which allows us to break early if
required.

Userland/Libraries/LibWeb/HTML/HTMLImageElement.cpp

@@ -864,6 +864,7 @@ static void update_the_source_set(DOM::Element& element)
         elements.clear();
         element.parent()->for_each_child_of_type<DOM::Element>([&](auto& child) {
             elements.append(&child);
+            return IterationDecision::Continue;
         });
     }