[DX] Introduce a coding style + tool

[Kocal]

Hi,

Using a coding style in an open-source projet is important, it allows code wrote by many people to stay consistent.

I don't know what PHP coding style you use at CKEditor, but if you don't, then using PSR-12 can be a nice start.

Using Easy Coding Standard is a really nice tool that combines both PHP-CS-Fixer and PHP_CodeSniffer, and provides some configuration sets.

For example, on my fork, I've used PSR12, PHPUnit, and clean code sets: https://github.com/Kocal/ckfinder-symfony-bundle/blob/main/ecs.php#L20-L24

WDYT?
Thanks!