Improve $$err typing (and fix doc)

[Gurimarukin]

• If the transformation is not possible, $$text expands to "null", not undefined. Corrected doc accordingly.

---

• You should be able to write this:

function $$textSafe(exp: unknown): string {
  const $res = $$text!(exp)

  if ($res === 'null') {
    return $$err!('$$text: exp in not transformable.')
  }

  return $res
}

But I get an error: Updating, $$err return type to never fixes the problem.

[GoogleFeud]

Thanks for the PR! Will be merged soon