LibWeb: Stop putting the FormattingState nodes in a slow hash map

Instead, put them in a Vector<OwnPtr<NodeState>>. Each layout node
has a unique index into the vector. It's a simple serial ID assigned
during layout tree construction. Every new layout restarts the sequence
at 0 for the next ICB.

This is a huge layout speed improvement on all content.

Userland/Libraries/LibWeb/Layout/Node.cpp

@@ -23,6 +23,8 @@ Node::Node(DOM::Document& document, DOM::Node* node)
     : m_document(document)
     , m_dom_node(node)
 {
+    m_serial_id = m_document->next_layout_node_serial_id({});
+
     if (m_dom_node)
         m_dom_node->set_layout_node({}, this);
 }