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

[snatvb]

Thanks for the PR! Will be merged soon

now feb 2025 and "soon" is the pr still opened
project is supporting or is it done?

no judge, just curious make sense to use it or not