Feature request: `setTimeout`

[wch]

I know that setTimeout is not implemented in the V8 engine itself; it is expected that the runtime which embeds V8 would implement setTimeout. In the case of this R package, it would be very useful to have setTimeout, since many JS libraries expect it to be present.

As for implementation, here are some possibilities:

• The later package could be used. A call to setTimeout() could use later's C++ API to schedule a callback that runs in V8.
• It would be even better if V8 could be run in a separate thread; when async code resolves, it could post messages back to the main R thread, again using the later package.