These APIs allow transferring a `JSObject` between worker threads. The
`JSObject.transfer` method creates a `JSObject.Transferring` instance
that is `Sendable` and can be sent to another worker thread. The
`JSObject.receive` method requests the object from the source worker
thread and postMessage it to the destination worker thread.

...mple

This change makes the transferring object to be used with a typed object
like `JSDate` or something else in the future.