LibWeb: Implement HTMLDetailsElement's open attribute closer to the spec

The spec now has a "toggle task tracker" to coalesce rapid changes to
this attribute. It also now has an explicit ToggleEvent to encapsulate
the old and new state of the element.

This further handles the attribute being added/removed using a override
of Element::attribute_changed(), rather than being the only element to
instead override Element::set/remove__attribute().

Userland/Libraries/LibWeb/HTML/ToggleTaskTracker.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <AK/String.h>
+#include <LibWeb/HTML/EventLoop/Task.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/interaction.html#toggle-task-tracker
+struct ToggleTaskTracker {
+    // https://html.spec.whatwg.org/multipage/interaction.html#toggle-task-task
+    // NOTE: We store the task's ID rather than the task itself to avoid ownership issues.
+    Optional<int> task_id;
+
+    // https://html.spec.whatwg.org/multipage/interaction.html#toggle-task-old-state
+    String old_state;
+};
+
+}