add playground demo

strutil/string.go

@@ -283,7 +283,7 @@ func SplitEx(s, sep string, removeEmptyString bool) []string {
 }
 
 // Substring returns a substring of the specified length starting at the specified offset position.
-// Play: Todo
+// Play: https://go.dev/play/p/q3sM6ehnPDp
 func Substring(s string, offset int, length uint) string {
 	rs := []rune(s)
 	size := len(rs)