LibWeb: Visit custom element's lifecycle callbacks

...instead of using JS::Handle which causes leaks when object holding
the callback can be reached by visiting the callback's dependencies.

Userland/Libraries/LibWeb/HTML/CustomElements/CustomElementDefinition.cpp

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2024, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/HTML/CustomElements/CustomElementDefinition.h>
+
+namespace Web::HTML {
+
+void CustomElementDefinition::visit_edges(Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    for (auto& callback : m_lifecycle_callbacks)
+        visitor.visit(callback.value);
+}
+
+}