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.cpp

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/MapIterator.h>
+
+namespace JS {
+
+MapIterator* MapIterator::create(GlobalObject& global_object, Map& map, Object::PropertyKind iteration_kind)
+{
+    return global_object.heap().allocate<MapIterator>(global_object, *global_object.map_iterator_prototype(), map, iteration_kind);
+}
+
+MapIterator::MapIterator(Object& prototype, Map& map, Object::PropertyKind iteration_kind)
+    : Object(prototype)
+    , m_map(map)
+    , m_iteration_kind(iteration_kind)
+    , m_iterator(map.entries().begin())
+{
+}
+
+MapIterator::~MapIterator()
+{
+}
+
+void MapIterator::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(&m_map);
+}
+
+}