Improve code quality

[FelixJacobi]

• Strict types, where missing (could break compatibility)
• Return type hints, where still missing (must be done in a major release, since it breaks compatibility unlike parameter type hints)

[FelixJacobi]

Rector?

[SamuelWei]

Can psalm also do this, as we already have it installed

[FelixJacobi]

Yes, it can partly, but it's mainly focused on missing types in code. Rector is more focused on modernizing code in general. Using IServ-GmbH/cotor: The Composer Tools Installer we can just install it and play it around with it ^^ . I will work on this in the next days.