Add named scope inversion with "not"

Nobody has claimed this yet.

I'm wanting to write a named scope like this:

```cr
class TaskQuery < Task::BaseQuery
def incomplete
completed_at.is_nil
end

def complete
not.incomplete
end
end
```

Is it possible to invert named scope results? Or is the best practice here to keep it explicit with completed_at.is_not_nil?

This is a relatively common convention for tracking boolean states with timestamps like approval, completion, acceptance, etc, so it'd be really handy to make that a touch easier!

Start by tracing named-scope behavior from Task::BaseQuery and the incomplete/complete examples in the issue. Define done as a tested, supported way to invert a named scope with `not`, or a clear project-supported alternative if inversion is not intended.

Written by the indexing model from the issue text.