As well rename test dir to tests, as Pest does not support dir without s on the end.
Also, remove PHP requirement from composer.json as we already have check-in bin/dep and it's better to have the check only in one place (now they already out of sync).
* its invalid to combine `void` with another type
see https://github.com/phpstan/phpstan/issues/4014
* added explicit `return null`, to make phpstan happy and the code more obvious.
* increased phpstan level to 5
* fix: '$subject of function preg_match expects string, int given'
