update project to ember 3.28

[miguelcobain]

• updates the project to 3.28 (the latest 3.x version).
• updates tests
• updates syntax everywhere (native classes, no get(), etc)
• adds github actions tests
• the recommended usage is now:

  @groupBy('model', 'color')
  carsByColor;

[miguelcobain]

@HeroicEric can you give me access to npm management of this package, please? My username is miguelcobain. Much appreciated.

[miguelcobain]

Thank you!