Support for factorial

[NhanAZ]

Something like this

$parser = Parser::createDefault();
$expression = $parser->parse("2! + 3!");
var_dump($expression->evaluate()); // int(8)

or

$parser = Parser::createDefault();
$expression = $parser->parse("fact(2) + fact(3)");
// https://www.php.net/manual/en/function.gmp-fact.php
var_dump($expression->evaluate()); // int(8)

2! + 3! instead of 1*2 + 1*2*3