Request: Move to Dart Sass from LibSass

[francisbarton]

Follow-up / branch issue from #27 - LibSass will be deprecated and certain preferred features such as @use (instead of @import are only available in the Dart Sass implementation.
(see here for example).

[wch]

This is definitely on our radar. There are some technical roadblocks in the way, however. With LibSass, we can include the C library in the package, which we know will run on all platforms. With Dart Sass, it requires a JavaScript runtime, which is a bit more challenging to do run on all platforms.