feat: format calendars

[redryerye]

Summary

• Add public ICFormatter.format(_:) support for serializing calendars to .ics text
• Serialize core calendar fields and supported VEVENT fields
• Reuse TEXT escaping and line folding so generated output is parser-compatible
• Mark calendar formatting complete in the README roadmap

Example ICS

Given an ICalendar with an event, formatting can produce output like:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Example Inc//Calendar//EN
CALSCALE:GREGORIAN
BEGIN:VEVENT
UID:format-test
DTSTAMP:20240728T120000Z
DTSTART;VALUE=DATE:20240729
SUMMARY:Board\, product\; roadmap
DESCRIPTION:Line 1\nLine 2
END:VEVENT
END:VCALENDAR

What becomes available

let ics = ICFormatter.format(calendar)
let parsed = ICParser().calendar(from: ics)

The formatter currently covers supported calendar fields and supported VEVENT fields, including TEXT escaping, DATE/DATE-TIME values, URI values, text lists, and folded content lines.

Validation

• swift test
• swiftlint lint --quiet