Add option to use ember-power-select.scss

[mkszepp]

Add option for consumer apps to use import @import 'ember-power-select.scss'

this works with ember-cli-scss only by adding scssOptions

let app = new EmberApp(defaults, {
    ...
    sassOptions: {
      includePaths: ['node_modules/ember-power-select'],
    },
    ...
  });

This was maybe working in v7 (without adding the setting sassOptions), but was not documented