Implement dynamic return type using phpstan conditional return types instead of phpstan extension

[tacovandenbroek]

Dynamic return typing was implemented using a phpstan extension (see: #94 for instance). Currently this could be implemented using "Conditional return types", like this: https://phpstan.org/r/97cd0a13-6c55-4473-ae96-5ac94807cbde

This prevents the need to use all phpstan rules from Safe (we do not want to migrate completely, yet), but still let phpstan know what the return types are.

Also, using native features is always preferable over extending imho 😉