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/Layout/BlockFormattingContext.cpp

@@ -1215,6 +1215,7 @@ CSSPixels BlockFormattingContext::greatest_child_width(Box const& box) const
         box.for_each_child_of_type<Box>([&](Box const& child) {
             if (!child.is_absolutely_positioned())
                 max_width = max(max_width, m_state.get(child).margin_box_width());
+            return IterationDecision::Continue;
         });
     }
     return max_width;