Pass promise argument to Promise._unhandledRejectionFn

[MattiasBuelens]

Currently, Promise._unhandledRejectionFn only receives the rejection reason err. However, in order to implement a "faithful" replica of the global unhandledrejection event, this also needs to receive the unhandled Promise itself.

This adds the promise as a second argument for Promise._unhandledRejectionFn. The default implementation doesn't use this argument, but users could replace it with their own version, for example:

Promise._unhandledRejectionFn = function(reason, promise) {
  globalThis.dispatchEvent(new PromiseRejectionEvent('unhandledrejection', { reason, promise }));
};