Migrate deprecated @import rules

Sass `@import` rules are deprecated and will be removed in Dart Sass 3.0.0.

More info and automated migrator: https://sass-lang.com/d/import

Note that this issue is not an urgent thing to resolve, the version `3.0.0` will not be released soon, as even the version `2.0.0` hasn't been released yet. Maintainers expect the version `3.0.0` to be released in approximately 2 years.