Add PHPStan Github action

[rvanlaak]

I'm in favor of just merging this PR, and decide on fixing / adding baseline later.

[Nyholm]

Could you also add a baseline?

[rvanlaak]

Done, added baseline.

And learned something new Today. Apparently twig/twig should have been added as dev dependency, as PHPStan otherwise isn't able to parse that. In other words, PHPStan also helps with finding hidden missing dependencies 🎉

[Nyholm]

Cool!
Thank you

[Nyholm]

Should be on push and pull_request.

[rvanlaak]

I still didn't find a situation where solely push didn't run the tests. Adding them both on the contrary will run the tests twice when you open a PR.

What about leaving it for now? I'll file a PR if it causes problems 👍

[Nyholm]

If you do a PR from a fork, then this will action will never run.

Feel free to merge this now and do a PR from a fork later to try yourself.