Support scheduled / cron triggers in standalone workerd

It would be useful if standalone workerd could natively schedule scheduled() handlers, similar to Cloudflare Workers Cron Triggers.

For example, allowing cron expressions to be configured in config.capnp:

```
scheduled = [
(
service = "worker",
cron = "*/5 * * * *"
)
]
```

This would make self-hosted Workers with periodic background jobs possible without requiring an external cron/systemd/Kubernetes scheduler.

Would this be something workerd could support?

Start by reading config.capnp and tracing how standalone workerd handles scheduled() handlers. Define how the example cron configuration should be represented and loaded, then verify that configured schedules invoke the worker periodically without an external scheduler.

Written by the indexing model from the issue text.