feat: parse vfreebusy components

[redryerye]

Summary

• Add an ICFreeBusy model for parsed VFREEBUSY components
• Parse common free/busy fields including UID, DTSTAMP, DTSTART, DTEND, ORGANIZER, FREEBUSY, and URL
• Parse FREEBUSY values into ICPeriod entries
• Expose parsed free/busy components on ICalendar.freeBusy
• Mark VFREEBUSY parsing complete in the README roadmap

Example ICS

BEGIN:VFREEBUSY
UID:freebusy-test
DTSTAMP:20240728T120000Z
DTSTART:20240729T090000Z
DTEND:20240729T170000Z
ORGANIZER:mailto:organizer@example.com
FREEBUSY:20240729T100000Z/20240729T110000Z,20240729T130000Z/PT30M
END:VFREEBUSY

What becomes available

let freeBusy = calendar.freeBusy.first
freeBusy?.uid // "freebusy-test"
freeBusy?.freeBusy?.first?.start
freeBusy?.freeBusy?[1].duration?.minutes // 30

Validation

• swift test
• swiftlint lint --quiet