LibWeb: Introduce the slottable concept for DOM elements and text nodes

A slottable is either a DOM element or a DOM text node. They may be
assigned to slots (HTMLSlotElement) either automatically or manually.
Automatic assignment occurs by matching a slot's `name` attribute to
a slottable's `slot` attribute. Manual assignment occurs by using the
slot's (not yet implemented) `assign` API.

This commit does not perform the above assignments. It just sets up the
slottable concept via IDL and hooks the slottable mixin into the element
and text nodes.

Userland/Libraries/LibWeb/DOM/Slottable.cpp

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Bindings/MainThreadVM.h>
+#include <LibWeb/DOM/Element.h>
+#include <LibWeb/DOM/Node.h>
+#include <LibWeb/DOM/ShadowRoot.h>
+#include <LibWeb/DOM/Slottable.h>
+#include <LibWeb/DOM/Text.h>
+#include <LibWeb/HTML/HTMLSlotElement.h>
+
+namespace Web::DOM {
+
+SlottableMixin::~SlottableMixin() = default;
+
+void SlottableMixin::visit_edges(JS::Cell::Visitor& visitor)
+{
+    visitor.visit(m_assigned_slot);
+    visitor.visit(m_manual_slot_assignment);
+}
+
+// https://dom.spec.whatwg.org/#dom-slotable-assignedslot
+JS::GCPtr<HTML::HTMLSlotElement> SlottableMixin::assigned_slot()
+{
+    // FIXME: The assignedSlot getter steps are to return the result of find a slot given this and with the open flag set.
+    return nullptr;
+}
+
+}