Enforce Prettier defaults + also format .babelrc and .json (#7499)

* .prettierrc: full configuration for Prettier

This way, contributors using different-from-default settings will still
use the correct prettier settings

* Makefile: also lint .babelrc.js

* Makefile: also prettify .json files

* Exclude package.json files and correct build dir

* Add more default options to prettierrc and load it in cli

This avoids prettier looking up the config for each file

* Format json

* Update prettier and eslint and reformat codebase

* Remove obsolete file

* Add comment

test/warning.js

@@ -1,2 +0,0 @@
-console.error("Don't run `mocha` directly. Use `make test`. See CONTRIBUTING.md for usage instructions.");
-process.exit(0);