Implement PHPStan

Static code analysis via PHPStan → will complement other measures intended to catch errors and implicit assumptions, e.g. #3174603: Add automated tests → , #3305740: Use strict typing in all PHP files (declare(strict_types=1);) → , #3305735: Type hint method/function parameters, return types, and class properties → , etc.

See core #3178534: Start running PHPStan on Drupal core (level 0) → .