Adds the \Stringable interface to classes that have __toString() method (#5920)

Co-authored-by: 李铭昕 <715557344@qq.com>

Author: huangdijia

src/Functions.php

@@ -20,13 +20,13 @@
 function str($string = null)
 {
     if (func_num_args() === 0) {
-        return new class() {
+        return new class() implements \Stringable {
             public function __call($method, $parameters)
             {
                 return Str::$method(...$parameters);
             }
 
-            public function __toString()
+            public function __toString(): string
             {
                 return '';
             }