replace render with createRoot

[gavindoughtie]

See: reactwg/react-18#5

In a next.js/React18 application, seeing this warning:

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. Learn more: https://reactjs.org/link/switch-to-createroot
    at RichMarkdownEditor (webpack-internal:///./node_modules/rich-markdown-editor/dist/index.js:100:9)