fix: allow ZoomableView ref to be utilised via ZoomableViewWithGestures

[mrbrentkelly]

Fixes #94.

Updates the ReactNativeZoomableViewWithGestures component to manage the ref of the inner ReactNativeZoomableView component.

This allows users of ReactNativeZoomableViewWithGestures to still imperatively call the zoomTo, zoomBy, moveBy, moveTo functions of the underlying ReactNativeZoomableView.

Example:

const ref = useRef<ReactNativeZoomableViewWithGestures>(null);

return <>
  <ReactNativeZoomableViewWithGestures {ref} ... />
  <Button text="Zoom" onPress={() => {
    ref.current?.zoomableViewRef?.current?.zoomBy(2);
  }} />  
</>

[elliottkember]

Nice one – thank you!