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`.
Adds an import map of the core and ext JavaScript files. This was
created manually but a script to create one automatically wouldn't be
too much of a big thing either.
This should make working on especially the Node polyfills much more
pleasant, as it gives you feedback on if your imports are correct.
Unfortunately the TypeScript declaration files of some of the internal
modules clash with the import map and override the data from the actual
files with data from the declaration files. Those do not contain all
exports nor is their data always up to date. Still, this is much better
than not having one.
