feat(cron) implement Deno.cron()

[igorzi]

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.

[littledivy]

Is saffron maintained anymore? cloudflare/saffron#20

Anyways, we should ask saffron to upgrade to nom 7.1

[lucacasonato]

I got confirmation that Cloudflare is happy to merge PRs, as it is still in use at CF.

[igorzi]

@littledivy can we do this after this PR is landed?

[littledivy]

Sure

[Hexagon]

Working on an alternative to saffron that might be of interest - with second granularity and support for timezoned DateTimes. Also nicknames for @hourly. Extended syntax like L for last occurrence of weekday in month. Standards-compliant weekday entry (1 is monday, in saffron 0 seem to be monday). Early stuff, started yesterday, and (disclaimer) my first Rust project ever, but starting to look like something - https://crates.io/crates/croner/0.0.3 https://github.com/hexagon/croner-rust

[Hexagon]

@igorzi ready to run (or to be roasted 😉):

https://github.com/Hexagon/croner-rust/releases/tag/v1.0.0

https://github.com/Hexagon/croner-rust#why-croner-instead-of-cron-or-saffron

[losfair]

Maybe use a different error message, Invalid cron name for a duplicate name is confusing - e.g. a dependency of dependency registers a cron and suddenly you get this error in your own code

[igorzi]

Oh, this test is wrong. 2nd arg to assertThrows should be the exception type. I'll fix the tests and make sure that it's checking for the correct error message.

[igorzi]

fixed

[losfair]

LGTM