Replace StyleCI with PHPCS

StyleCI needs a pretty long time to get compatible with new PHP syntax features and/or versions.
To don't fail the CI until SCI is up2date we should switch to PHPCS which upgrades faster.

We could also install https://github.com/tighten/duster but should use the Laravel preset: https://gist.github.com/laravel-shift/cab527923ed2a109dda047b97d53c200 (possible PR to duster as new preset).

Start by locating the repository's CI configuration and current StyleCI setup, then review PHPCS and the optional Duster Laravel preset mentioned in the issue. Done means StyleCI is replaced, PHPCS runs successfully in CI, and checks remain compatible with newer PHP syntax and versions.

Written by the indexing model from the issue text.