add Result.withDefault Result.mapError simplification

[morteako]

Result.withDefault x (Result.mapError f result)
--> Result.withDefault x result

[lue-bird]

perfect :)

[jfmengels]

Agreed, this is great! Thank you very much @morteako 🙏

For future PRs, feel free to ignore committing docs.json, that makes commits less noisy and also avoids Git conflicts.

If you're interested in continuing on adding Result simplifications, there's a few proposals in #287 (some may have been done already, I haven't checked yet). Obviously, no pressure, just a nudge 😁