fix(ios): add countdown to UIDatePickerMode converter (#804)

Author: Nasseratic

ios/RNDateTimePickerManager.m

@@ -25,6 +25,7 @@ @implementation RCTConvert(UIDatePicker)
   @"time": @(UIDatePickerModeTime),
   @"date": @(UIDatePickerModeDate),
   @"datetime": @(UIDatePickerModeDateAndTime),
+  @"countdown": @(UIDatePickerModeCountDownTimer),
 }), UIDatePickerModeTime, integerValue)