Add completion for format conversion specifications

[King2500]

echo sprintf('Hello %<caret>');

https://www.php.net/manual/en/function.printf.php https://www.php.net/manual/en/function.sprintf.php https://www.php.net/manual/en/function.fprintf.php https://www.php.net/manual/en/function.vprintf.php https://www.php.net/manual/en/function.vsprintf.php https://www.php.net/manual/en/function.vfprintf.php https://www.php.net/manual/en/function.sscanf.php https://www.php.net/manual/en/function.fscanf.php