setCurrentSelection method
Sets both endpoints of the "current" selection
Implementation
CalendarState setCurrentSelection(Date a, Date? b,
{CausedBy cause = CausedBy.rangeConfirm,
bool previewAnchoredAtStart = false}) =>
setSelection(CalendarSelection.guessOrder(currentSelection, a, b),
cause: cause, previewAnchoredAtStart: previewAnchoredAtStart);