Generalize `is_constructor` to `can_return` and also have a check for `-> void`

[KANAjetzt]

          i'd still generalize `is_constructor` to `can_return` and also have a check for `-> void` being present. it's currently completely fine to return a method even if that method is statically typed (see image). but i would assume this will be fixed at some point, so we should not neglect it. 

topic for a follow up pr

Originally posted by @Qubus0 in #515 (review)

[KANAjetzt]

closed by #518