React.useRef to handle ReactDOMRe.domRef

reasonreact.re

React.useRef to handle ReactDOMRe.domRef

Here are a few solutions:
```ocaml
let inputRef = React.createRef();
<input ref={ReactDOMRe.Ref.domRef(inputRef)} type="text" />
```
If React.createRef is not available in your current version, simply use:
```ocaml
let inputRef = React.useRef(Js.Nullable.null);
<input ref={ReactDOMRe.Ref.domRef(inputRef)} type="text" />
```
In case that you want to convert Js.Nullable.t to option beforehand:
```ocaml
let inputRef = React.useRef(None);
<input ref=ref={ReactDOMRe.Ref.callbackDomRef(elem =>
    React.Ref.setCurrent(inputRef, Js.Nullable.toOption(elem))
)} type="text" />
```