Use GitHub actions instead of Travis CI

[sschmid]

GitHub added Actions to automate.

Migrate from Travis CI to GitHub actions.

Current Travis CI: https://travis-ci.org/github/sschmid/Entitas-CSharp

• [x] Run unit tests on push
• [x] Add automated release

[sschmid]

Doing CI/CD with GitHub actions now using these steps:

• build
• test
• publish
• generate and upload coverage report
• attach assemblies, nuget packages and coverage report to workflow runs