LibJS: Add AsyncGenerator / AsyncGeneratorPrototype

Not implementing any prototype functions yet, but stubbing out async
generator infrastructure will allow us to make some progress in that
direction.

Userland/Libraries/LibJS/Runtime/AsyncGeneratorRequest.h

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Completion.h>
+#include <LibJS/Runtime/PromiseReaction.h>
+
+namespace JS {
+
+// 27.6.3.1 AsyncGeneratorRequest Records, https://tc39.es/ecma262/#sec-asyncgeneratorrequest-records
+struct AsyncGeneratorRequest {
+    Completion completion;        // [[Completion]]
+    PromiseCapability capability; // [[Capability]]
+};
+
+}