Added php-cs-fixer and php-parallel-lint

[roelofr]

This should make further contributions easier to verify, and more consistent.

The php-cs-fixer config is slightly opinionated towards modern code (PSR12 + strict_types).
It also adds a few rules about unit tests, to ensure consistent naming.

php-cs-fixer is auto-applied on the master branch, so new contributor don't have to worry about styling, It'll be fixed for them.