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.cpp

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Streams/AbstractOperations.h>
+#include <LibWeb/Streams/ReadableStream.h>
+
+namespace Web::Streams {
+
+// https://streams.spec.whatwg.org/#is-readable-stream-locked
+bool is_readable_stream_locked(ReadableStream const& stream)
+{
+    // 1. If stream.[[reader]] is undefined, return false.
+    if (stream.reader() == nullptr)
+        return false;
+
+    // 2. Return true.
+    return true;
+}
+
+}