Add RFC 5545 timezone interoperability to dateutil recurrence parsing

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What this task asks for

Extend rrule and rruleset to serialize, parse, and compare RFC 5545 timezone-aware recurrence data. Extend python-dateutil's rrule module with RFC 5545 timezone interoperability. RDATE gains TZID/VALUE parameter support. rrule and rruleset gain timezone-aware __str__, equality/hash/repr, property accessors, iCalendar serialization, and set operations. rrulestr gains VCALENDAR auto-detection with VTIMEZONE parsing and a tzids parameter. Rruleset.__str__() outputs DTSTART (from the first rrule), then RRULE, RDATE, EXRULE, EXDATE in order. Timezone-aware RDATE/EXDATE include TZID; UTC uses Z. EXRULE lines use the EXRULE: prefix. Read-only properties rrule.dtstart, rrule.freq, rrule.interval, rrule.until expose recurrence parameters. Rrule.count() returns the count parameter directly when set, otherwise iterates (inherited from rrulebase). Rrulestr auto-detects BEGIN:VCALENDAR, extracts VTIMEZONE and VEVENT. Only recurrence properties (DTSTART, RRULE, RDATE, EXRULE, EXDATE) from the first VEVENT. RFC 5545 line unfolding is handled. Inline VTIMEZONE definitions take priority over tzids lookups.