LibJS: Implement Iterator.prototype.map

This uses a new Iterator type called IteratorHelper. This does not
implement IteratorHelper.prototype.return as that relies on generator
objects (i.e. the internal slots of JS::GeneratorObject), which are not
hooked up here.

Userland/Libraries/LibJS/Runtime/IteratorHelper.cpp

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/IteratorHelper.h>
+#include <LibJS/Runtime/IteratorOperations.h>
+#include <LibJS/Runtime/Realm.h>
+
+namespace JS {
+
+ThrowCompletionOr<NonnullGCPtr<IteratorHelper>> IteratorHelper::create(Realm& realm, IteratorRecord underlying_iterator, Closure closure)
+{
+    return TRY(realm.heap().allocate<IteratorHelper>(realm, realm.intrinsics().iterator_helper_prototype(), move(underlying_iterator), move(closure)));
+}
+
+IteratorHelper::IteratorHelper(Object& prototype, IteratorRecord underlying_iterator, Closure closure)
+    : Object(ConstructWithPrototypeTag::Tag, prototype)
+    , m_underlying_iterator(move(underlying_iterator))
+    , m_closure(move(closure))
+{
+}
+
+void IteratorHelper::visit_edges(Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_underlying_iterator.iterator);
+}
+
+Value IteratorHelper::result(Value value)
+{
+    if (value.is_undefined())
+        m_done = true;
+    return value;
+}
+
+ThrowCompletionOr<Value> IteratorHelper::close_result(Completion completion)
+{
+    m_done = true;
+    return *TRY(iterator_close(vm(), underlying_iterator(), move(completion)));
+}
+
+}