Integration testing

[limarkxx]

Adds integration testing, with a sample multi-root workspace

[tm1000]

ooooooooo!

[tm1000]

I love laravel as much as the next person (I daily drive it) but I wonder if adding Laravel here adds too much bloat to this extension's git repo. Shouldnt we test this extension against some base PHP files instead?

Same applies with Slim or others.

[limarkxx]

Maybe but it should at least be something at least slightly challenging for psalm to munch on.
Perhaps put those files in https://github.com/psalm/sample-laravel-app and https://github.com/psalm/sample-slim-webapp, and use git submodules to bring them in as necessary?