[v4] Use new runtime by default or fix the detection

Created by: gaearon

I'm starting to think that automated detection might be a significant footgun. At least when done this way. https://github.com/facebook/create-react-app/pull/9788/files#r506932998

I'm leaning towards this proposal:

1. Enable new transform by default in V4 (since we've now released compatible React versions for all majors)
2. Offer an env option to turn it off

This also addresses automatic detection issues in https://github.com/facebook/react/issues/20031#issuecomment-710821817.