react-ts: make sure react-scripts works, use new jsx runtime

[fvsch]

This PR updates the react-ts template with the following goals in mind:

1. Use the new JSX runtime introduced in 2020, that doesn't require importing the React namespace. This coding style is used by the new React docs, by create-react-app@latest, and more.

2. Make sure that the project can be downloaded an ran locally with npm install && npm start. In my testing, react-scripts:

   • Required a tsconfig.json to be present to accept importing TypeScript files.
   • Required the package.json to have a browserslist config. By default it will add a verbose one, so to keep things compact I went with "browserslist": ["defaults"].

I also considered replacing react-scripts with vite. That would be my preference, by Vite uses conventions like an index.html at the root (not in a public folder), copying the public folder as-is, having the index.html request resources like styles and scripts, etc. All conventions which work well in WebContainer projects, but not in EngineBlock projects.

[sulco]

Thank you! 🙏