LibWeb: Implement ReadableStream's constructor

Author: https://github.com/mattco98
Commit: d8710aa604
Pull-request: https://github.com/SerenityOS/serenity/pull/18109
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/Streams/ReadableStream.h

@@ -32,7 +32,7 @@ public:
         Errored,
     };
 
-    static WebIDL::ExceptionOr<JS::NonnullGCPtr<ReadableStream>> construct_impl(JS::Realm&);
+    static WebIDL::ExceptionOr<JS::NonnullGCPtr<ReadableStream>> construct_impl(JS::Realm&, Optional<JS::Handle<JS::Object>> const& underlying_source);
 
     virtual ~ReadableStream() override;