[v4] Change JSX runtime detection mechanism to presence of /jsx-runtime.js

Created by: gaearon

https://github.com/facebook/create-react-app/pull/9645 detects 17+ which is not ideal because we're planning to release 16.x, 15.x, and 0.14.x releases with the new JSX runtime included. Ideally we should instead look at whether react/jsx-runtime.js exists.