Expose placeholder prop on Picker

[lyahdav]

This exposes the ComboBox PlaceholderText as a prop on Picker called placeholder.

Microsoft Reviewers: Open in CodeFlow

[NickGerleman]

@lyahdav the built-in Picker component is close to being jettisoned from react-native, and react-native-windows by proxy. E.g. the RNTester example for it was recently removed after the component has been deprecated for a few releases.

I'd definitely reccomend making any changes to the Picker community module, which supports Windows and has a very similar implementation. Not sure if FB's build system for RNW Would make that challenging, but it's likely this change will otherwise go away when the component is removed.

[NickGerleman]

I think @rectified95 might have some additional context

[NickGerleman]

Link to the community module. It looks like it does also have macos support https://github.com/react-native-picker/picker

[rectified95]

I agree with Nick - this code is only here because it has not been removed from RN core yet after deprecation. The community module repo is the way to go - the change should map 1-1 there.