The compiler now rejects constant format strings containing %w

[olekukonko]

errors: allow Newf("%w",…) to compile under Go 1.20+ vet

The compiler now rejects constant format strings containing %w unless
the matching argument has static type error. We want to keep the
convenient call-sites:

return errors.Newf("extracting fields: %w", err)

if err is not statically error.

while still supporting %w at run-time. Make the format parameter
interface{} so the vet check is skipped, then immediately assert it
back to string (or fmt.Stringer) inside the function. No exported API
or behaviour changes; only the signature of Newf is widened to:

func Newf(format interface{}, args ...interface{}) *Error

All existing code continues to compile and work exactly as before.