Support JSON schedule syntax for Deno Cron

https://deno.com/blog/v1.39#cron
https://deno.land/api@v1.39.0?s=Deno.CronSchedule&unstable=

```ts
Deno.cron("myCron", {
  minutes: { every: 20 },
}, () => {
  console.log("Running every 20 minutes");
});
```