[4.2] Add Non Latin Character Support For Str::slug

Add support for UTF-8 encoded slugging.  In case of slugging persian characters (like آ, ب, ژ) converting encoding to ASCII causes an empty string.

```php
// سلام-دنیا
Str::lightSlug('سلام دنیا'); 

//empty string 
Str::slug('سلام دنیا');
```