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/AsyncGenerator.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Variant.h>
+#include <LibJS/Runtime/AsyncGeneratorRequest.h>
+#include <LibJS/Runtime/ExecutionContext.h>
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+// 27.6.2 Properties of AsyncGenerator Instances, https://tc39.es/ecma262/#sec-properties-of-asyncgenerator-intances
+class AsyncGenerator final : public Object {
+    JS_OBJECT(AsyncGenerator, Object);
+
+public:
+    enum class State {
+        SuspendedStart,
+        SuspendedYield,
+        Executing,
+        AwaitingReturn,
+        Completed,
+    };
+
+    explicit AsyncGenerator(Object& prototype);
+    virtual ~AsyncGenerator() override = default;
+
+private:
+    virtual void visit_edges(Cell::Visitor&) override;
+
+    // At the time of constructing an AsyncGenerator, we still need to point to an
+    // execution context on the stack, but later need to 'adopt' it.
+    using ExecutionContextVariant = Variant<ExecutionContext, ExecutionContext*, Empty>;
+
+    Optional<State> m_async_generator_state;               // [[AsyncGeneratorState]]
+    ExecutionContextVariant m_async_generator_context;     // [[AsyncGeneratorContext]]
+    Vector<AsyncGeneratorRequest> m_async_generator_queue; // [[AsyncGeneratorQueue]]
+    Optional<String> m_generator_brand;                    // [[GeneratorBrand]]
+};
+
+}