Implements $cancel() method for ExtendedTask

[shikokuchuo]

Closes #4093.

Adds a $cancel() method to ExtendedTask, analogous to the python implementation.

This keeps it simple, and is straightforward as the do.call() in the code creates the task object anyway, and we just need to store it in a private field.

Previously I'd thought that the promise itself might have a $cancel method, but looking at javascript promises, this actually doesn't exist. It's also not natural to implement it that way, as promises::promise() takes functions as arguments, rather than the task object itself. We'd most likely need to rely on each as.promise() method to create this.

We could make cancellation extendable via a S3 generic for the task object for example, but as mirai is the only known implementation with cancellation capability thus far, we might defer this.