LibHTML: Add Document::get_element_by_id() and get_elements_by_name()

These will be useful for implementing various things. They don't do any
caching at the moment, but that might become valuable in the future.

To facilitate this change, I also made it possible to abort a tree walk
with for_each_in_subtree() by returning IterationDecision::Break from
the callback.

Libraries/LibHTML/DOM/Node.cpp

@@ -77,6 +77,7 @@ void Node::invalidate_style()
     for_each_in_subtree([&](auto& node) {
         if (is<Element>(node))
             node.set_needs_style_update(true);
+        return IterationDecision::Continue;
     });
     document().schedule_style_update();
 }