Implement nom-like `escaped_transform` Parser

See https://docs.rs/nom/latest/nom/bytes/complete/fn.escaped_transform.html for reference.

This allows the parser to return a unescaped version of an string.
E.g. "\\n" ->"\n"

Maybe also adding `escaped` function would be great.

Start with the linked nom escaped_transform reference and inspect the repository's parser-combinator entry points to understand how transformed string results are represented. The issue is done when escaped input such as "\\n" produces an unescaped newline; determine separately whether the optional escaped function belongs in scope.

Written by the indexing model from the issue text.