LibWeb: Give Element a CustomStateSet, exposed by ElementInternals

Author: https://github.com/AtkinsSJ
Commit: b6ffea8990
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5302
Reviewed-by: https://github.com/tcl3 ✅

Libraries/LibWeb/HTML/ElementInternals.h

@@ -57,6 +57,7 @@ public:
     WebIDL::ExceptionOr<bool> report_validity() const;
 
     WebIDL::ExceptionOr<GC::Ptr<DOM::NodeList>> labels();
+    GC::Ptr<CustomStateSet> states();
 
 private:
     explicit ElementInternals(JS::Realm&, HTMLElement& target_element);