fix(types): include optional nativeEvent.target on picker events

[cpruijsen]

Summary

Adds optional target?: number to DateTimePickerEvent and DateTimePickerChangeEvent (TypeScript and Flow) and documents it. Runtime event construction is unchanged: Android still omits the field; iOS dismiss still omits it.

Fixes #927.

iOS onChange / onValueChange events include nativeEvent.target (the React tag React Native injects on native view events); Android dialogs omit it, because the picker is not a view. The shared TypeScript type only listed timestamp and utcOffset, so accessing target was a type error even though iOS returns it. Reporter: compact mode on iOS, Expo SDK 51, intersecting DateTimePickerEvent with { nativeEvent: { target?: number } } to compile.

The types use optional target on the shared event nativeEvent rather than platform-specific event types (IOSDateTimePickerEvent vs AndroidDateTimePickerEvent) or stripping iOS extras so both sides match the old smaller type. Flow already models nativeEvent as an open object; optional target is the smallest TypeScript equivalent, matches the reporter's workaround, and keeps Android object literals assignable. Stripping iOS target would break the reporter. Can switch to platform-specific event types if that is preferred.

Test Plan

What's required for testing (prerequisites)?

A TypeScript consumer of DateTimePickerEvent. No device required for the type change; yarn test test/types.test.js typechecks a fixture under tsc --strict.

What are the steps to reproduce (after prerequisites)?

1. Assign onChange as (event: DateTimePickerEvent) => { event.nativeEvent.target }.
2. Before: TS2339 Property 'target' does not exist.
3. After: target is number | undefined. An Android-shaped event { type: 'set', nativeEvent: { timestamp, utcOffset } } still typechecks.

Optional on a simulator: compact iOS picker, log event.nativeEvent on change: target is present; Android dialog change: target is absent.

Compatibility

OS	Implemented
iOS	✅
Android	✅

Checklist

• I have tested this on a device and a simulator
• I added the documentation in README.md
• I updated the typed files (TS and Flow)
• I added a sample use of the API in the example project (example/App.js)
• I have added automated tests, either in JS or e2e tests, as applicable