Optimize the PHPStan configuration

Problem/Motivation

PHPStan is configured in a way that making small changes to the code instantly yields dozens of warnings, which then have to be added manually to the huge (1000+ lines) list of ignored warnings in phpstan-baseline.neon.

This is counterproductive and prevents developers from contributing.

Proposed resolution

• Adopt a more sensible PHPStan configuration.
• Fix all warnings that are fixable instead of ignoring them.