extract and expose recurrence rule for such events

[Serphentas]

This PR implements support for recurring events, by extracting recurrence rules and exposing them when available.

• Adds isRecurring: boolean to the event object
• Adds recurrenceRules: Array<{frequency, interval, daysOfTheWeek?, daysOfTheMonth?, monthsOfTheYear?, endDate?, count?}> to the event object
• Enables downstream tools to generate proper entries instead of creating individual occurrences, such as with CalDAV using RRULE