LibWeb: Start fleshing out the ReadableStream interface

This is so we can just assume it exists in Fetch APIs (while still
skipping functionality that relies on a full implementation, of
course).

Userland/Libraries/LibWeb/Streams/AbstractOperations.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Forward.h>
+
+namespace Web::Streams {
+
+bool is_readable_stream_locked(ReadableStream const&);
+
+}