[Babel 8] Set babelrc: false by default

[nicolo-ribaudo]

We have two types of configuration files: babel.config.js and .babelrc.js (docs).

.babelrc files are very expensive to load, because we must check if they are present in the file's folder or in any parent folder. They are also rarely used, because for 99% of use cases babel.config.js is enough. To save on I/O cost, we should only load them if the babelrc option is explicitly set to true.

However, this change has a disadvantage: some people like to keep their config in package.json, and we threat it as if it was a .babelrc file. Maybe we should add a rootConfig: true option in package.json#babel, to make it behave like a babel.config.json?