add `initialDate` to `MacosDatePicker`

[Luckey-Elijah]

Use case

I'd like to control the initial date set in MacosDatePicker widget.

Proposal

Create a parameter initialDate that sets _MacosDatePickerState._initialDate on init state. If the value of the parameter initialDate is null, then use DateTime.now().

Example of ideal usage:

MacosDatePicker(
  style: DatePickerStyle.textual,
  initialDate: DateTime(2022, 10, 10),
  onDateChanged: _onDateChanged,
)

[Luckey-Elijah]

Feel free to assign to me for a contribution :smile: