add `whenRef(ref)` variant for `eb.case(ref)`.

Hey 👋 

We support `when(lhs, op, value)` and `when(val)`.
We'll support `whenRef(lhs, op, ref)` in 0.29.

We're missing support for the following:

```ts
eb.case('is_ready')
  .when(false)
  .then('abort')
  .whenRef('is_set') // <<---------
  .then('go!')
  .else('abort')
  .end()
```