Using GitHub actions as CI would help with contributing and pull-request

[Stefan Bürk]

This package contains some unit- and integration tests.
Sadly, they are not executed automatically on pushes and
for pull-requests, which opens for the possibility to miss
broken tests.

GitHub action workflow file should be added executing at least:

• plain php linter to check for invalid php syntax
• execute unit tests
• execute integration test

against supported laravel and php versions.

In further follow-ups this may be extended to use other database
backends for the integration test, and not only sqlite.