Implement fireEvent API

[thymikee]

See https://github.com/kentcdodds/react-testing-library#fireeventeventnamenode-htmlelement-eventproperties-object.

In RN world we could use it like

fireEvent.press(button);
fireEvent.doublePress(button);
fireEvent.changeText(textInput);
fireEvent.scroll(scrollView);

I'd like this to only have events that can be triggered by the user on core components.
Since any "basic" component like Picker, Checkbox, RadioButton can (and usually is) be implemented in user-land, I don't think there's room for so many events as in DOM world.

Accessing props directly would still be possible, because we probably shouldn't care about underlying native implementation (e.g. for gesture handlers)