Add cancel-on-interrupt behavior

[DavisVaughan]

See posit-dev/positron#8002

Needs "the major internal backend design started in future (>= 1.40.0)"

[HenrikBengtsson]

See future.apply for an example. The gist is that one have to handle interrupts both when creating futures and when waiting for their values. When an interrupt or an error is detected, one signals cancel(futures) on all active futures. I have helper functions onInterrupt() and onError() for this. Since onInterrupt() signals an error, the onError() handler will take care of the canceling;

https://github.com/futureverse/future.apply/blob/develop/R/condition-handlers.R

Then I catch interrupt and error in my internal xapply() helper function and call those handlers;

https://github.com/futureverse/future.apply/blob/4f7d42f96fef5519a68c634f2d2a50a30041daff/R/future_xapply.R#L303C1-L307C21

That should be it.

[HenrikBengtsson]

I can attack this after #286 and #287 have been resolved.