refactor(ir): make `ops.SortKey` a non-value operation

[kszucs]

Since aliases are valid in many places but asc()/desc() are really only valid in order_by() I wonder if asc()/desc() should return non Value operations? This would make it easier to forbid e.g. t.x.asc() + 1.

Originally posted by @jcrist in https://github.com/ibis-project/ibis/issues/8100#issuecomment-1912256153