* fix: appveyor test failure
* ci: bring back yarn cache
* ci: turn off matrix
* refactor: use babel instead of esm in jest
* refactor: use es modules in fixtures
After #3564 and the request of #3427 (plus #3452) I decided to add a dedicated `vueConfig`object to the `nuxt.config.js` file.
The keys will be projected on Vue.config.
By default, `Vue.config.performance` is enabled in dev mode, `Vue.config.silent` is enabled in production mode.
Doc PR incoming.
Resolves: #2910, #3427
