Create site for experimenting with cron expressions

Currently the only way to experiment with cron expressions for Cron Triggers is to use the Workers dashboard. I think it'd be nice if there existed another site that simply let users experiment with cron expressions, maybe even letting the user view all future execution times. This would probably be done on the `cron-triggers.cloudflareworkers.com` zone since we already have the backup worker there, so we'd just build the site on top of it.