Use Sass modules

[hartsick]

At the end of 2019 Sass announced moving to modules, a move away from @import statements. We should do the same, and migrate to modules / @use.

Timeline link
One year after both Dart Sass and LibSass have launched support for the module system or two years after Dart Sass launches support for the module system, whichever comes sooner (1 October 2021 at latest), we will deprecate @import as well as global core library function calls that could be made through modules.

One year after this deprecation goes into effect (1 October 2022 at latest), we will drop support for @import and most global functions entirely. This will involve a major version release for all implementations.

Split out of #260