Add phpstan configuration file

[tmotyl]

This PR provides basic configuration file to run phpstan on openMage code base.
Right now it just whitelists few core modules to show that it's working.
allowing more modules requires validation and probably some fixes, so should be tackled in separate PR's.

TODO:
Help is needed in providing a github action which will run phpstan on every PR.

Description (*)

Provides basic configuration for running phpstan

Related Pull Requests

Fixed Issues (if relevant)

Fixes:

1. Fixes Add ability to run PHPStan  #365

Manual testing scenarios (*)

1. pull
2. composer install
3. run vendor/bin/phpstan analyze -c .github/phpstan.neon

Questions or comments

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All automated tests passed successfully (all builds are green)
• Add yourself to contributors list

[Flyingmana]

I created a new PR as I want this changes directly in the lowest version branch.
In this step I also added it to our github actions with an optional run, which executes it with a higher level, and also for a non yet covered path (so we always have a current overview of what things need to improve for the next steps.)

[Flyingmana]

merged via #1875 1875