Define @@setter with class_getter

[Blacksmoke16]

Reboot of #345. Was getting:

Showing last frame. Use --error-trace for full trace.

In lib/granite/src/granite/select.cr:19:5

19 | @@select
^
Error: can't infer the type of class variable '@@select' of Granite::Base.class

The type of a class variable, if not declared explicitly with
`@@select : Type`, is inferred from assignments to it across
the whole program.

...

Other assignments have no effect on its type.

can't infer the type of class variable '@@select' of Granite::Base.class

When working on some Athena stuff.