Expose recurrence option constants and centralize validation

[ggaabe]

Summary

• export reusable runtime option lists: allowedFreq, allowedWeekdays, plus Weekday and weekdayToIsoDay
• centralize weekday parsing/mapping and FREQ/WKST validation in core rule parsing/sanitization
• reuse shared weekday constants in totext
• add tests for exported lists and new normalization/validation behavior
• document reusable option lists in README

Verification

• npm run build
• npm test