LibJS: Implement async functions as generator functions in BC mode

This applies a simple transformation, and adds a simple wrapper that
translates the generator interface to the async function interface.

Userland/Libraries/LibJS/Runtime/FunctionKind.h

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2021, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace JS {
+
+enum class FunctionKind {
+    Generator,
+    Regular,
+    Async,
+};
+
+}