LibJS: Add the Map built-in object

Userland/Libraries/LibJS/Runtime/Map.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/HashMap.h>
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS {
+
+class Map : public Object {
+    JS_OBJECT(Map, Object);
+
+public:
+    static Map* create(GlobalObject&);
+
+    explicit Map(Object& prototype);
+    virtual ~Map() override;
+
+    HashMap<Value, Value, ValueTraits> const& entries() const { return m_entries; };
+    HashMap<Value, Value, ValueTraits>& entries() { return m_entries; };
+
+private:
+    virtual void visit_edges(Visitor& visitor) override;
+
+    HashMap<Value, Value, ValueTraits> m_entries; // FIXME: Replace with a HashMap that maintains a linked list of insertion order for correct iteration order
+};
+
+}