Add Cow-based internal tokenizer functions for zero-copy preparation

  Add internal _borrowed() functions that return Cow<\'a, str> to prepare for
  zero-copy tokenization. When the source string needs no transformation
  (no escaping), return Cow::Borrowed. When transformation is required,
  return Cow::Owned.

  The Token enum still uses String, so borrowed values are converted via
  to_owned() for now. This maintains API compatibility while preparing the
  codebase for a future refactor where Token can hold borrowed strings.

  Optimized: comments, quoted strings, dollar-quoted strings, quoted identifiers.

Author: eyalleshem