[php-src] Issue #22695: Nullable Casts

Issue: https://github.com/php/php-src/issues/22695
Author: rela589n

### Description

Hi!

From time to time, I come across the desire to have nullable casts: `(?string)$value`.

It should work as follows:

```php
$cast = (?string)$value;
$cast = nullableString($value);
```

```php
function nullableString(mixed $value): ?string
{
    if (null === $value) {
        return null;
    }

    return (string)$value;
}
```