Increase phpstan level to 8 with baseline to swallow existing errors

[Seldaek]

Closes #661 - this implements it in a different way, instead of a sub-directory with the phpstan binary we just require phpstan in the CI, so it runs there, but can't easily be run locally (well you have to composer require phpstan/phpstan marc-mabe/php-enum-phpstan --dev && git checkout composer.json then you can call vendor/bin/phpstan in your dev env).

It's a compromise situation until legacy php versions are dropped, which probably would be reasonable to do for 6.0 IMO, but another discussion for another day.

[DannyvdSluijs]

@Seldaek any change you can do a git rebase master && git push --force-with-lease to trigger the GitHub Actions?
I'm planning to merge this as part of the next version

[Seldaek]

Ok one sec