Add initTimezones()

[Levi-Lesches]

Using TZDateTime correctly is a core part of this package. As such, the user needs to understand a bit of how package:timezone works. While writing the new usage guide, I realized that there is only really one correct way to do this, and based on the example, I think it looks like this:

import 'package:flutter_timezone/flutter_timezone.dart';
import 'package:timezone/data/latest_all.dart' as tz;
import 'package:timezone/timezone.dart' as tz;

void initTimezones() {
  // Not supported or needed on desktop and web platforms
  if (!Platform.isAndroid && !Platform.isIOS && !Platform.isMacOS) return;
  tz.initializeTimeZones();
  final timeZoneName = await FlutterTimezone.getLocalTimezone();
  tz.setLocalLocation(tz.getLocation(timeZoneName!));
}

From this point on, users can simply use tz.local to get their current tz.Location. So maybe instead of making everyone copy this snipper and adding both packages, maybe we can offer this as a top-level function and only make users include package:timezone, but not need to use getLocation and setLocalLocation themselves.