Implement `getNativeScrollRef`

[j-piasecki]

This PR adds implementation for getNativeScrollRef. It's required for compatibility with react-native-reanimated on the new architecture: https://github.com/software-mansion/react-native-reanimated/blob/720fefba7cd53471e98d52282c783701233861a7/src/reanimated2/hook/useAnimatedRef.ts#L26-L33

[ananyachandra14]

Getting Typescript errors, can you check? @j-piasecki

[j-piasecki]

Now I remember why it was unknown 😅 in the first place:

• ScrollViewComponent didn't exist yet in React Native 0.49 and this is the version of @types/react-native used and React.ElementRef doesn't seem to exist in React 16
• I didn't want to import anything from React Native in web-specific section of the code

I solved it by:

• using ScrollView as a return type which, from admittedly short testing, seems to be forward-compatible
• declaring ScrollView type to be unknown for web

[ananyachandra14]

Thanks @j-piasecki!