)
Creates a new repeating timer.
The callback is invoked repeatedly with duration intervals until
canceled with the cancel function.
The exact timing depends on the underlying timer implementation.
No more than n callbacks will be made in duration * n time,
but the time between two consecutive callbacks
can be shorter and longer than duration.
In particular, an implementation may schedule the next callback, e.g.,
a duration after either when the previous callback ended,
when the previous callback started, or when the previous callback was
scheduled for - even if the actual callback was delayed.