Fix naming of rollup plugins and helper functions in ember-scoped-css

[candunaj]

Renamed some helper functions.
Exposed only one unplugin scoped-css-unplugin. It will be easier to use in applications.

import { scoped-css-unplugin } from 'ember-scoped-css';

export default defineConfig({
 plugins: [
  ...,
  addon.keepAssets(...),
+  scopedCssUnplugin.rollup(),
]
});

[mansona]

Looks great! 🎉