CSS imports

[sgratzl]

based on #29

parses @import './test.css' lines in CSS files and adds them as imports within rollup,

e.g.,

@import './test.css';
@import '~test_module/test.css';
.root {
  color: red;
}

will create the following intermediate JS module

import './test.css';
import 'test_module/test.css'; // note the ~ was removed similar how css-loader is handling imports
export default '';

Thus, triggers import the imported files during bundling.

[thgh]

Can you further discuss this with @jerekshoe and merge it into the v4 branch?