LibWeb: Add PopOverInvokerElement and use it in HTMLButtonElement

The popoverTargetElement seems to be one of the only cases of a
reflected Element? attribute in the HTML spec, the behaviour of which
is specified in section 2.6.1.

Buttons can't actually toggle popovers yet because showing/hiding
popovers is not implemented yet.

Libraries/LibWeb/HTML/PopoverInvokerElement.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2024, Nathan van der Kamp <nbvdkamp@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGC/Ptr.h>
+#include <LibJS/Heap/Cell.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::HTML {
+
+class PopoverInvokerElement {
+
+public:
+    PopoverInvokerElement() { }
+
+    GC::Ptr<DOM::Element> get_popover_target_element() { return m_popover_target_element; }
+
+    void set_popover_target_element(GC::Ptr<DOM::Element> value) { m_popover_target_element = value; }
+
+protected:
+    void visit_edges(JS::Cell::Visitor& visitor)
+    {
+        visitor.visit(m_popover_target_element);
+    }
+
+private:
+    GC::Ptr<DOM::Element> m_popover_target_element;
+};
+
+}