Migrate to Sass module system

[eritbh]

Announcement post and @use documentation

Sass has released a new module system that solves a lot of edge cases and issues that arose from repurposing @import for Sass files as well as local and remote CSS files. It would probably be beneficial to update the sass lib used in this project to a version that supports the new module system (Dart Sass 1.23.0 is listed as the first compatible version in the blog post). There's also a migration tool available that might help out here, though I haven't tested how well it works.

If I get some free time soon I may try to tackle this myself, the biggest challenge will probably just be ensuring that the version of Sass used for CI builds is correct.

[eritbh]

blocked on #69, libsass doesn't support @use yet

[eritbh]

#77 un-blocked this, soon:tm: