Set up Github actions (especially unit tests)

[therouv]

Set up Github actions and run unit tests for all supported Magento / PHP versions to provide early feedback to developers if everything still works as expected.

Unit tests should run on push to main branch and every pull request:

• https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushbranchestagsbranches-ignoretags-ignore
• https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpull_requestpull_request_targetbranchesbranches-ignore

Discovered https://github.com/extdn/github-actions-m2 during a quick research. Maybe a good starting point?