Add phpstan checks

[Nyholm]

I want to try @OskarStark’s github action: https://github.com/OskarStark/phpstan-ga

What do you think? Should we try to see if phpstan can help us while reviewing PRs?

[OskarStark]

Nice 👍

Let me know when I can support you, but first we need ti find out, if mailgun organization is invited to the beta (and is currently available for "organizations" 🤔 )

[Nyholm]

Oh, I thought it was just to make a PR like #530 and it would magically turn up in the PR's "Checks".

[OskarStark]

You are right, from this moment, where Github will allow/activate Github Actions for everyone or at least public organization repositories it will work

[OskarStark]

As I am using FROM phpstan/phpstan:0.10 docker image as base, it is not possible to use phpstan extensions.