LibWeb: Introduce the slot concept for HTML slot elements

A slot is an HTMLSlotElement. It may have any number of slottable nodes
assigned to it.

Userland/Libraries/LibWeb/DOM/Slot.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/String.h>
+#include <LibWeb/DOM/Slottable.h>
+
+namespace Web::DOM {
+
+// https://dom.spec.whatwg.org/#concept-slot
+class Slot {
+public:
+    virtual ~Slot();
+
+    String const& slot_name() const { return m_name; } // Not called `name` to distinguish from `Element::name`.
+    void set_slot_name(String name) { m_name = move(name); }
+
+    ReadonlySpan<DOM::Slottable> assigned_nodes_internal() const { return m_assigned_nodes; }
+    void set_assigned_nodes(Vector<DOM::Slottable> assigned_nodes) { m_assigned_nodes = move(assigned_nodes); }
+
+protected:
+    void visit_edges(JS::Cell::Visitor&);
+
+private:
+    // https://dom.spec.whatwg.org/#slot-name
+    String m_name;
+
+    // https://dom.spec.whatwg.org/#slot-assigned-nodes
+    Vector<Slottable> m_assigned_nodes;
+};
+
+}