Right now, Command::execute() returns int|void.

I'd suggest we should deprecate returning void (or null) by throwing a deprecation when the method returns a non-int, in Application.

We would need to update all core commands to return 0; where they return; right now.

And in 5.0, we would make returning a non-int throw an exception, in preparation for adding a real return type in 6.0.

Help wanted.