Add phpstan to static code analysers

[DvDty]

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[streichsbaer]

Thanks for the PR @DvDty.

While phpstan is a static code analyzer, it doesn't support security rules as far as I know.
This awesome-php-security list is only focused on security-related resources.

Let me know if I missed something and phpstan actually has security rules, then I'll merge the PR.

Otherwise, I have to close it.