add an ember demo package to validate use w/in Ember applications

[tomwayson]

Recently I've noticed that tree shaking in not working for these packages in our Ember app, which uses ember-auto-import to bring in Hub.js packages. Possibly related: embroider-build/ember-auto-import#121

Our Ember app is complex, with many engines, and a challenging environment to debug. It would be good to add an clean Ember app here to validate what we expect from ember-auto-import and eventually embroider (possibly it's own demo).