feat: add support for second level field in cron, 6-field cron

[CakeCrusher]

https://github.com/CakeCrusher/exo/blob/9beecd6518f00fda63e22f7cda0dec786a121301/exo/harness.ts#L82

I know you can also @every <N>s but this wont sync with wall clock.
For more requent or higher time resolution processes second field becomes useful.
With the incoming Astra GPT 6 model this sub minute level precision will become increasingly useful.