Introduce GitHub Actions config
Introduce GitHub Actions config

Travis is dead, so this needs to happen anyway. Key things to notice:

- The Travis config remains because this version of the GHA config does
  not have the publishing step set up; that still needs to be moved
  over.

- The Travis-powered CI is currently failing, and also does not run
  against *any* recent version of Ember, and this intentionally does
  not update what is tested; a future change should do that.

kategengler/ember-feature-flags,kategengler/ember-feature-flags