allow css-loader to resolve @imports

[magsallen]

We've run into a problem in the next-myft-page and next-article Page Kit migrations where the sass-loader plugin is unable to resolve imports with .css extensions. x-dash components output their styles as css so importing these stylesheets currently throws errors.

Setting this property in the cssLoaderOptions allows the css-loader to resolve these imports. Their contents can then be added to the concatenated public/styles.css bundle.

An alternative solution would be to update this regex so that the sass-loader is able to resolve .css files at that point instead.

Thoughts?

[i-like-robots]

Just a note to say that the required behaviour should have been possible previously. Sass can lookup and include .css files as other Sass partials so long as the import meets the criteria here: https://sass-lang.com/documentation/at-rules/import#plain-css-imports

e.g.:

@import 'path/to/css-styles'; // will lookup file as a partial and try to include contents
@import 'path/to/css-styles.css';  // will compile to `@import url(path/to/styles.css);`

If it didn't work as shown above then we may need to raise a bug with an upstream dependency. See sass/libsass#754 also for more info about the original implementation.