Updates to styles imported from JS cause a full reload

[kategengler]

When editing styles that are ultimately imported into app.js, a full reload of the app happens, rather than the hot loading of styles that would occur with the traditional pipeline for styles.

Not sure if this is exactly an embroider bug, or a symptom of the approach (which is necessary when using embroider due to #1358).

In my project, we are using scss, which necessitates style-loader, etc, but I was able to reproduce this with a clean app generated on v5.11 of ember-source with --embroider with only css.

https://github.com/kategengler/repro-styles