Fix PHPStan and enable it in CI

[vasilvestre]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Fixed tickets	https://github.com/FriendsOfSylius/SyliusImportExportPlugin/issues/255

Help wanted

• There's some design issue with type, how should we handle this ?
• phpstan/phpstan-symfony should fix command argument type problems but it do not