LibWeb: Defer invalidation sets processing until update_style()

This change converts `Node::invalidate_style()` (invalidation sets
overload) from eagerly doing tree traversal that marks elements affected
by invalidation set to instead adding "pending invalidation sets" into
`StyleInvalidator`, processing of which is deferred until the next
`update_style()`. By doing that we sometimes substantially reduce amount
of work done performing tree traversal that marks elements for style
recalculation.

Improves performance on Discord, were according to my measurements we
were previously spending 20% of time in style invalidation, but now it's
down to <1%.

Libraries/LibWeb/DOM/Document.h

@@ -903,6 +903,8 @@ public:
 
     String dump_display_list();
 
+    StyleInvalidator& style_invalidator() { return m_style_invalidator; }
+
 protected:
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;