WorkersCron.prototype.on(expression: string,handler: CronHandler): this
Registers a handler for a cron expression.
Several handlers may share one expression; all of them run, and one that
rejects does not prevent the others. The expression is matched exactly
against ScheduledController.cron, so it must be written the same way it
appears in wrangler.toml.
handler: CronHandler
Invoked when that trigger fires