Add php-cs-fixer GitHub workflow

[simensen]

I'm not exactly sure if this makes sense, but if there is PHP code in this package, I guess we can use the rules for this package itself somehow? :)

[zaherg]

To be honest, I am not sure what the workflow will do, except those two in my head:

1. check the code style 🫣

At the end the only thing we can test here is that it returns the correct rules we have set