LibWeb: Support Document.onvisibilitychange

We don't have a way to trigger this from a test, but the fix is so
simple that we might as well get it in. :^)

(cherry picked from commit 274411db9756a9dffe44e21b4da09c7808e9f465)

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -5637,4 +5637,14 @@ void Document::set_onreadystatechange(WebIDL::CallbackType* value)
     set_event_handler_attribute(HTML::EventNames::readystatechange, value);
 }
 
+WebIDL::CallbackType* Document::onvisibilitychange()
+{
+    return event_handler_attribute(HTML::EventNames::visibilitychange);
+}
+
+void Document::set_onvisibilitychange(WebIDL::CallbackType* value)
+{
+    set_event_handler_attribute(HTML::EventNames::visibilitychange, value);
+}
+
 }

Userland/Libraries/LibWeb/DOM/Document.h

@@ -722,6 +722,9 @@ public:
     [[nodiscard]] WebIDL::CallbackType* onreadystatechange();
     void set_onreadystatechange(WebIDL::CallbackType*);
 
+    [[nodiscard]] WebIDL::CallbackType* onvisibilitychange();
+    void set_onvisibilitychange(WebIDL::CallbackType*);
+
 protected:
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;

Userland/Libraries/LibWeb/DOM/Document.idl

@@ -152,7 +152,7 @@ interface Document : Node {
 
     // special event handler IDL attributes that only apply to Document objects
     [LegacyLenientThis] attribute EventHandler onreadystatechange;
-    [FIXME] attribute EventHandler onvisibilitychange;
+    attribute EventHandler onvisibilitychange;
 };
 
 dictionary ElementCreationOptions {