Add manual for mb_str_pad function since PHP 8.3

ref: https://wiki.php.net/rfc/mb_str_pad

```
function mb_str_pad(string $string, int $length, string $pad_string = " ", int $pad_type = STR_PAD_RIGHT, ?string $encoding = null): string {}
```

example:

```php
var_dump(mb_str_pad("🎉", 3, "祝", STR_PAD_LEFT)));
```

result:

```
string(10) "祝祝🎉"
```