Support `yearAndMonth` mode on iOS

[thisisgit]

Feature request

Hi, I just found out that iOS date picker supports year&month mode starting 17.4 and this isn't supported in this library yet.

Why it is needed

There were already some people requesting for this feature but I believe it was not possible at the time:

• Is it possible to make a date picker only with month and year? #23
• Feature Request : Mode to select only month & year from the calendar #521
• is there way we can have year selection? or month and year selection not date?  #632

As a workaround, I tried copying it with two react-native-pickers but there're some limitations where

1. selectionColor can't go transparent in the latest version (how can i set the background color to transparent react-native-picker/picker#242 (comment))
2. Has different style from the year & month mode of UIDatePicker, e.g. gap between items & font styles
3. Has limited number of rows even height of the picker is increased

I was able to somehow solve 1&2 by sticking to the older version(2.6.0) and adjusting the styles by changing font size and scaling the whole picker down, but 3 is something that can't be solved due to limitation of the picker itself.

Possible implementation

I'm not really familiar with native code but I guess adding one more entry in the enum here would at least make it work?

datetimepicker/ios/RNDateTimePickerManager.m

Lines 24 to 29 in 53033d2

	RCT_ENUM_CONVERTER(UIDatePickerMode, (@{
	@"time": @(UIDatePickerModeTime),
	@"date": @(UIDatePickerModeDate),
	@"datetime": @(UIDatePickerModeDateAndTime),
	@"countdown": @(UIDatePickerModeCountDownTimer),
	}), UIDatePickerModeTime, integerValue)

But overall, I think following needs to be handled:

• Type support and validation on the JS side for yearAndMonth mode
• Map yearAndMonth entry in iOS native code (above)
• Use fallback mode in Android if yearAndMonth is passed
• Use fallback mode or warn in iOS if yearAndMonth is passed and iOS version is below 17.4