Strongly typed ref in BabylonNode<T>

[coder89]

Fixes typescript error for a given case:

const ref = useRef<Mesh>();
useEffect(() => {
  const mesh = ref.current;
}, []);
return <mesh ref={ref} />

Without that change one needs to write below code to workaround type check errors which defeats the purpose:

const ref = useRef();
useEffect(() => {
  const mesh = ref.current as Mesh;
}, []);
return <mesh ref={ref} />

[brianzinn]

thanks Lukasz!! That's a helpful addition 😄