This PR changes the `Deno.cron` API:
* Marks the existing function as deprecated
* Introduces 2 new overloads, where the handler arg is always last:
```ts
Deno.cron(
name: string,
schedule: string,
handler: () => Promise<void> | void,
)
Deno.cron(
name: string,
schedule: string,
options?: { backoffSchedule?: number[]; signal?: AbortSignal },
handler: () => Promise<void> | void,
)
```
This PR also fixes a bug, when other crons continue execution after one
of the crons was closed using `signal`.
This PR adds unstable `Deno.cron` API to trigger execution of cron jobs.
* State: All cron state is in memory. Cron jobs are scheduled according
to the cron schedule expression and the current time. No state is
persisted to disk.
* Time zone: Cron expressions specify time in UTC.
* Overlapping executions: not permitted. If the next scheduled execution
time occurs while the same cron job is still executing, the scheduled
execution is skipped.
* Retries: failed jobs are automatically retried until they succeed or
until retry threshold is reached. Retry policy can be optionally
specified using `options.backoffSchedule`.
