Babel: Only change browserslist defaults if no config file exists

[danez]

https://github.com/babel/babel/pull/8897#issuecomment-490659840

I guess the best solution would be to check ourself if there is a browserlist config in the fs if there are not targets specified. We could do this with browserlist.findConfig(from). If we find one we do not change defaults. But that would also be a breaking change so I guess we would need to wait for v8. https://github.com/browserslist/browserslist/blob/master/node.js#L238

[taralx]

FYI it's "browserslist" and not "browserlist", which is why I did not originally find this one. :)