Add `pack`/`unpack` support for signed integers with specific endianness

### Description

The same way as already added to `float` according to the changelog https://www.php.net/manual/en/function.pack.php.

Currently code like this is needed:
```
$unpackToSignedIntLeFx = static function (string $v) {
    $unpacked = unpack('va/Cb/cc', $v);

    return ($unpacked['c'] << 24) | ($unpacked['b'] << 16) | $unpacked['a'];
};
```