feat: support %w verb in Errorf for error wrapping - #254

Why not just make fundamental implement Unwrap?

There are a couple of architectural considerations around fundamental and Unwrap:

1. Leaf errors vs. wrapped errors:
   fundamental represents a leaf error created by New(msg) containing only msg string and *stack. It holds no underlying error. If fundamental were given an Unwrap() error { return nil } method, it would be a no-op for chain traversal, as standard library errors.Is and errors.As already treat types without Unwrap (or returning nil) as terminal leaves.

2. Storing the wrapped error:
   To support Errorf("%w", err), the wrapped error operand must be retained in memory so Unwrap() can return it. If fundamental were expanded to include an err error field, every leaf error returned by New would incur an unused error field (16 bytes on 64-bit architectures).

3. Go 1.20 multi-error wrapping:
   Standard library fmt.Errorf supports multiple %w verbs (e.g., fmt.Errorf("%w %w", err1, err2)), returning a type that implements Unwrap() []error. In Go, a single struct cannot implement both Unwrap() error and Unwrap() []error due to conflicting method return signatures.
   Following the standard library pattern (fmt.wrapError and fmt.wrapErrors), separate types allow Errorf to return:

   • wrapError (Unwrap() error) for single %w
   • wrapErrors (Unwrap() []error) for multiple %w
   • fundamental for plain format strings without %w

That said, if you prefer to keep pkg/errors focused strictly on single-error wrapping and would rather unify single-wrap Errorf into fundamental (adding err error and Unwrap() error directly to fundamental), I am glad to adjust the implementation to match your preferred design.