fix: use %w in Prefix so errors.Is and errors.As work through prefixed errors

[amitmishra11]

Bug

Prefix wraps each constituent error (and non-multierror values) with
fmt.Errorf("%s %s", prefix, e). Using the %s verb formats the
original error into a plain string, discarding the error value. As a
result, errors.Is and errors.As cannot see through the prefixed
wrapper to reach the original error.

Reported in #56.

Reproduction:

sentinel := errors.New("sentinel")

// non-multierror path
result := multierror.Prefix(sentinel, "context:")
fmt.Println(errors.Is(result, sentinel)) // false -- wrong

// multierror path
multi := &multierror.Error{Errors: []error{sentinel}}
prefixed := multierror.Prefix(multi, "context:")
fmt.Println(errors.Is(prefixed, sentinel)) // false -- wrong

Fix

Replace %s with %w in both fmt.Errorf calls inside Prefix. The
%w verb wraps the original error so the standard library error chain
traversal can reach it. The formatted message is identical
("<prefix> <original message>"), so callers that only inspect the
string representation are unaffected.

Test

Added TestPrefix_ErrorsIs which asserts that errors.Is finds the
original sentinel through both the non-multierror and multierror paths of
Prefix.

All existing tests continue to pass.