LibJS: Add the SetIterator built-in and Set.prototype.{values, entries}

While this implementation should be complete it is based on HashTable's
iterator, which currently follows bucket-order instead of the required
insertion order. This can be simply fixed by replacing the underlying
HashTable member in Set with an enhanced one that maintains a linked
list in insertion order.

Userland/Libraries/LibJS/Runtime/SetIterator.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/HashTable.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/Set.h>
+
+namespace JS {
+
+class SetIterator final : public Object {
+    JS_OBJECT(SetIterator, Object);
+
+public:
+    static SetIterator* create(GlobalObject&, Set& set, Object::PropertyKind iteration_kind);
+
+    explicit SetIterator(Object& prototype, Set& set, Object::PropertyKind iteration_kind);
+    virtual ~SetIterator() override;
+
+    Set& set() const { return m_set; }
+    bool done() const { return m_done; }
+    Object::PropertyKind iteration_kind() const { return m_iteration_kind; }
+
+private:
+    friend class SetIteratorPrototype;
+
+    virtual void visit_edges(Cell::Visitor&) override;
+
+    Set& m_set;
+    bool m_done { false };
+    Object::PropertyKind m_iteration_kind;
+    HashTable<Value, ValueTraits>::Iterator m_iterator;
+};
+
+}