Made err() infer strings narrowly for easier error tagging

[mattpocock]

I would like err to infer strings more narrowly. This would make it easier to create unions out of possible string errors.

const example = (num: number) => {
  if (num < 0) return err('Too low');
  if (num > 100) return err('Too high');

  return ok(num);
}

This would infer as Ok<number> | Err<'Too low'> | Err<'Too high'>.

Let me know if this is desirable.

[changeset-bot]

🦋 Changeset detected

Latest commit: eadf50c

The changes in this PR will be included in the next version bump.

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Name	Type
neverthrow	Patch

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[mattpocock]

Can do this with an overloaded type if you'd prefer, but figured this was the easiest way to make this change.

[mattpocock]

Wasn't sure where this test should go, so made its own block.

[mattpocock]

Side note - prettier can't run on this file because it's an outdated version and there's an 'import type'.

When I did try to run prettier on it, a LOT of changes were made, probably to do with the semicolons.

[supermacro]

@m-shaka mentioned we may want to use biome instead of prettier, but perhaps if we just update prettier this issue goes away?

[supermacro]

Thanks for the contribution @mattpocock :)

[macksal]

Just found this in the blame while working on #584. Nice work @mattpocock. It might be useful for ok/okAsync/errAsync to have the same overload.