A program with no async functions, generators or timers never ran the
event loop, so a promise rejection nothing handled went unreported:
`Promise.reject(new Error('x'))` alone exited 0, where Node prints the
error and exits 1. The loop now also runs when the program makes a
promise (a Promise static, `new Promise` or Promise.withResolvers), and
its end reports the rejection, runs the 'exit' listeners with code 1 and
exits 1, as for async programs. Programs that make no promise keep their
code unchanged.
Chris Tate
7bb87229c2