[String] Snakecase breaked 

### Symfony version(s) affected

6.1.4

### Description

Since #47185, when using `u('GREAT')->snake()->toString()` it return us `g_reat` instead of `great`.

### How to reproduce

```php
u('GREAT')->snake()->toString();
```

### Possible Solution

_No response_

### Additional Context

_No response_