Add `@DartStatic`

[wilkomanger]

This means we won't generate static members by default anymore for objects.

Because every member name needs to be unique in Dart (and static and instance scope is shared), add an optional name: String argument to @DartStatic, to specify the generated static's name. By default this should be the instance member name with $ added in front. For companion objects, the default name can be exactly the same as the instance member name, because they won't be in the same scope.