Add GitHub Actions

[stefanzweifel]

This Pull Request shows how we add GitHub Actions to this repository:

1. 0ae7949: We add an empty Workflow File
2. 8490594: Add Action to install composer dependencies
3. 238aa80: Add Action to run our phpunit test suite
4. a973a43 & 9b428e8: Add phpinsights and execute insights in an Action
5. 61a8a48 & 1a52e01: Add php-cs-fixer and add Actions which run php-cs-fixer and commit possible fixed files back to the repository (see 1c20ab0).