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