LibWeb: Make "assign slottables for a tree" fast when there are no slots

We achieve this by keeping track of the number of HTMLSlotElements
inside each ShadowRoot (do via ad-hoc insertion and removal steps.)

This allows slottables assignment to skip over entire shadow roots when
we know they have no slots anyway.

Massive speedup on https://wpt.fyi/ which no longer takes minutes/hours
to load, but instead a "mere" 19 seconds. :^)

Libraries/LibWeb/HTML/HTMLSlotElement.cpp

@@ -8,6 +8,7 @@
 #include <LibWeb/Bindings/HTMLSlotElementPrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/DOM/Element.h>
+#include <LibWeb/DOM/ShadowRoot.h>
 #include <LibWeb/DOM/Text.h>
 #include <LibWeb/HTML/HTMLSlotElement.h>
 
@@ -147,4 +148,21 @@ void HTMLSlotElement::attribute_changed(FlyString const& local_name, Optional<St
     }
 }
 
+void HTMLSlotElement::inserted()
+{
+    Base::inserted();
+    auto& root = this->root();
+    if (!root.is_shadow_root())
+        return;
+    static_cast<DOM::ShadowRoot&>(root).increment_slot_count();
+}
+
+void HTMLSlotElement::removed_from(Node* old_parent, Node& old_root)
+{
+    Base::removed_from(old_parent, old_root);
+    if (!old_root.is_shadow_root())
+        return;
+    static_cast<DOM::ShadowRoot&>(old_root).decrement_slot_count();
+}
+
 }