Add error type to mark conflicts during apply

[bluekeyes]

Apply now returns an error with type *Conflict if the error is because of a conflict between the patch and the target. This allows callers to respond to conflicts differently from other errors.

The conflict error message now also includes the file path containing the conflict, which can help understand problems even if the client otherwise doesn't treat conflicts specially.

Detect any conflict

if errors.Is(err, &patch2pr.Conflict{}) {
  // respond to conflict
}

if conflict, ok := errors.AsType[*patch2pr.Conflict](err); ok {
  switch conflict.Type {
    // respond to different conflicts differently
  }
}

Detect specific conflict

if errors.Is(err, &patch2pr.Conflict{Type: patch2pr.ConflictModifiedFileMissing}) {
  // respond to conflict
}

Alternative approach to #165.