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/AsyncFunctionDriverWrapper.h

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2021, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Bytecode/Interpreter.h>
+#include <LibJS/Runtime/ECMAScriptFunctionObject.h>
+#include <LibJS/Runtime/GeneratorObject.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/Promise.h>
+
+namespace JS {
+
+class AsyncFunctionDriverWrapper final : public Promise {
+    JS_OBJECT(AsyncFunctionDriverWrapper, Promise);
+
+public:
+    static ThrowCompletionOr<Value> create(GlobalObject&, GeneratorObject*);
+    explicit AsyncFunctionDriverWrapper(GlobalObject&, GeneratorObject*);
+
+    virtual ~AsyncFunctionDriverWrapper() override;
+    void visit_edges(Cell::Visitor&) override;
+
+    ThrowCompletionOr<Value> react_to_async_task_completion(VM&, GlobalObject&, Value, bool is_successful);
+
+private:
+    GeneratorObject* m_generator_object { nullptr };
+    NativeFunction* m_on_fulfillment { nullptr };
+    NativeFunction* m_on_rejection { nullptr };
+};
+
+}