API: `getEvaluatable` on `stack` for execution/evaluation ops

### Describe the feature

The evaluation patterns (Hermes, Iris, Thoth) all use an `evaluatable(...` function rather than a `stack.getEvaluatable` method call, which differs from other common built-in Iota types and specializations (lists, numbers/ints, etc).

### Additional context

Xref: https://github.com/FallingColors/HexMod/pull/1106#discussion_r3376034809