Constructor de expresiones cron

AWS's dialect adds a year field and uses 1-7 for day-of-week (1=Sunday), not 0-6 like standard cron. It also requires exactly one of day-of-month/day-of-week to be ? — you can't leave both as *, and can't restrict both at once. Only numeric values/lists/ranges/steps and ? are supported here — the L/W/# special characters AWS also allows aren't parsed (rare enough in practice, and getting their date math wrong silently is worse than not supporting them), so an expression using them will be rejected rather than mis-evaluated.