confusable: ignore replacement characters

Normalizing the string will return those in some cases and they're
considered graphic, but they're not actually in the input string, so
they should be dropped from the skeleton.

confusable/skeleton.go

@@ -29,7 +29,7 @@ func Skeleton(input string) string {
 	var builder strings.Builder
 	builder.Grow(len(input))
 	for _, r := range input {
-		if !unicode.IsGraphic(r) {
+		if !unicode.IsGraphic(r) && r != 0xfffd {
 			continue
 		}
 		repl := GetReplacement(r)
@@ -48,7 +48,7 @@ func SkeletonBytes(input string) []byte {
 	var builder bytes.Buffer
 	builder.Grow(len(input))
 	for _, r := range input {
-		if !unicode.IsGraphic(r) {
+		if !unicode.IsGraphic(r) && r != 0xfffd {
 			continue
 		}
 		repl := GetReplacement(r)