added str_word_count to the stub file (fix #6016)

stubs/CoreGenericFunctions.phpstub

@@ -843,6 +843,17 @@ function strrchr(string $haystack, string $needle) {}
  */
 function strrev(string $string): string {}
 
+/**
+ * @psalm-pure
+ *
+ * @psalm-return (
+ *     $format is positive-int
+ *     ? array<string>
+ *     : int
+ * )
+ */
+function str_word_count(string $string, int $format = 0, string|null $characters = null) {}
+
 /**
  * @psalm-pure
  *