feat: support different calendars

[JonasWanke]

Closes: #13

Checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

[JonasWanke]

@Jamalianpour Using the persian calendar (or any other calendar) is now supported. As I don't know much about it though, there might still be bugs or unsuitable behavior — please let me know if you find any!

To use the persian calendar, specify it when constructing your TimetableController:

myController = TimetableController(
  calendar: CalendarSystem.persianSimple,
  // ...
);

All dates you pass to timetable must also use this calendar (most notably start and end of an Event and the initialDate property of TimetableController).

I discovered a bug in time_machine while developing this feature, so I'll wait for a fix (Dana-Ferguson/time_machine#35) to be released before publishing this to pub. You can still try this feature today by specifying the following in your pubspec.yaml:

dependencies:
  time_machine:
    git:
      url: https://github.com/JonasWanke/timetable.git
      ref: issue/13-support-different-calendars

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• This PR will close the following issues: support different calendars #13

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[Ahmadre]

@JonasWanke do you need help in maintaining this project?

[JonasWanke]

I'm currently working on the major refactor tracked in #17, but after that, I'd appreciate contributions!

Regarding this PR: Due to dropping the dependency on time_machine, support for different calendars probably won't come soon. See #17 (comment) for more information.