Wrong conversion to snake case from upper snake case

### Symfony version(s) affected

6.1.2

### Description

Symfony String converts `X_Y` into `xy` while I expect it to do `X_Y`. `X_Y` is in upper-snake case, the underscore should not be thrown away.

### How to reproduce

```php
\Symfony\Component\String\u('X_Y')->snake()
```

### Possible Solution

_No response_

### Additional Context

_No response_