Improve behavior around existing `styles` directory

[alexlafroscia]

• JS files within the styles directory should be available
  • This means that a JS file within the styles directory should be import-able
  • A JS file within the styles directory should be lookup-able through the resolver to support a use case where the style is not actually in the pod (file should still be able to be resolved)
• It should be possible not to have an app.css file at all

Prior Art

• ember-css-modules somehow allows this, I think. Probably can check that out for a reference