Support Nested Text Elements Inside of TextInput

[NickGerleman]

See TokenizedTextExample in TextInputSharedExamples.js. Nesting here seems supported by both Android + iOS and we currently just Yellowbox.

[chrisglein]

Works in Android with something like this:

      <TextInput>
        Hello <Text style={{fontSize: 24}}>embedded</Text> World!
      </TextInput>

Snack
Didn't work the same in iOS (formatting didn't show). Or in Web (although it was having an issue with TextInput - may be possible).

This is going to be hard to implement in XAML for sure.