Support React 18 createRoot in index.js

[sammyjava]

I find that I have to stick with the React 17 method of rendering the RootContainer for my tools to work in BG. Since Progress is Good, I'd like to be able to use the React 18 createRoot method as suggested by the 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

FWIW the demo works fine with createRoot, but integration into BG fails with an "Object() ... is not a function" error.