Previously, creating the `app` dir without enabling the config would
print an error about `pages` missing:
> Couldn't find a `pages` directory. Please create one under the project
root
This PR prints a better error message:
> The `app` dir is experimental. Please add `{experimental:{appDir:
true}}` to your `next.config.js` to enable it
Co-authored-by: Balázs Orbán <info@balazsorban.com>