[meta] Add scalar type hinting and return value typing

Well I violated my own request of not posting patches in a meta ... I opened a MR because the remaining missing parameter and return type hints are scattered all over the code - not in one compact place. I figure one MR that we can add to and have tested is now the easiest way to resolve the remaining bits in a quick and efficient manner.

There are probably some places we could use nullable type hints, but in general I'm not a fan of passing around null then having to costantly check if the value is null or not, so my preference is usually to enforce a type with a value and don't accept or return null unless we're interacting with core functions that give us no choice.

TR → credited chrisolof → .

This will also resolve the related issue I just linked.

Automatically closed - issue fixed for 2 weeks with no activity.