Add modern CI using GitHub Actions

[JeroenDeDauw]

This repo still has a TravisCI based setup which no longer works.

Instead use the same approach as https://github.com/ProfessionalWiki/SPARQL (Another example: https://github.com/ProfessionalWiki/WikibaseExport)

• [ ] Makefile
• [x] Tests run on GitHub Actions
• [ ] Code coverage via codecov.io (instead of Scrutinizer)
• [ ] Optionally adding static analysis with Psalm and/or PHPStan

[malberts]

Initial Github Actions CI added in https://github.com/ProfessionalWiki/ModernTimeline/pull/35