WIP: Expanded ical support

This branch contains a large update to the repo with breaking changes to the @rschedule/rschedule package.

• Updates @rschedule/ical-tools to depend on the ical.js package for parsing iCal strings and adds enhanced iCal string parsing / serializing functionality that more closely aligns with the spec.
  • Can now parse entire VCALENDAR and VEVENT components inside iCal strings
• Removes ical parsing / serializing functionality from @rschedule/rschedule and all associated objects.
  • In the future, you must include and use @rschedule/ical-tools to get parsing / serializing functionality.

Fixes #8 (closed), #9 (closed), #10 (closed)