LibWeb: Don't invalidate style for entire document on Node insertion

Like for attribute changes, we now only invalidate the insertion parent
and all of its descendants. Again, this is very aggressive, but also
way less than doing the entire document.

Once we implement the CSS :has() selector, we'll need to become more
sophisticated about invalidation.

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -458,7 +458,8 @@ void Node::insert_before(JS::NonnullGCPtr<Node> node, JS::GCPtr<Node> child, boo
     // 9. Run the children changed steps for parent.
     children_changed();
 
-    document().invalidate_style();
+    // FIXME: This will need to become smarter when we implement the :has() selector.
+    invalidate_style();
 }
 
 // https://dom.spec.whatwg.org/#concept-node-pre-insert