LibJS: Add the MapIterator built-in and the key/values/entries methods

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

Userland/Libraries/LibJS/Runtime/MapIterator.h

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