Helium package should be able to import css

Helium package should be able to import css. For example,

• Someone want to create spell using jsoneditor (https://github.com/josdejong/jsoneditor/blob/master/docs/usage.md). He / She will import like this

import 'jsoneditor/dist/jsoneditor.min.css'
import JSONEditor from 'jsoneditor/dist/jsoneditor.min.js'