Use createRoot from React 18 and render from react-dom/client

[psb]

Using

| Some(root) => ReactDOM.render(<App />, root) 

gives the warning:

ReactDOM.render is no longer supported in React 18. Use createRoot instead. Until you switch to the new API, your app will behave as if it's running React 17

Using

| Some(root) => {
        let app = ReactDOM.Experimental.createRoot(root);
        ReactDOM.Experimental.render(app, <App />);
      }

gives the warning:

Warning: You are importing createRoot from "react-dom" which is not supported. You should instead import it from "react-dom/client".

Remove Experimental module from src/ReactDOM.re and use createRoot from React 18 and render from react-dom/client.