Support dart-sass' `pkg:` importer

[TLATER]

Hi, dart-sass gained a cool feature that allows importing packages from various package managers by prefixing their url with pkg:.

To properly support this for node packages, the package.json file needs to list its sass/scss files explicitly in exporters - at least the index. Currently bulma doesn't do this, so relative imports with pkg: don't always resolve correctly. It'd be nice if that field could be added to package.json.