Replace libsass with Dart Sass CLI

[eritbh]

Among other things, this lets us take advantage of the modern Sass module system and @use directives. Un-blocks #63.

[eritbh]

Sass is run as npx -q sass FILENAME because for some reason Github Actions doesn't allow npm install --global, and without a global install, binaries distributed in npm packages aren't on PATH. The node_modules location in Actions can be weird, so we just run it through npx to avoid confusion. -q suppresses output from npx itself downloading the package in a fresh environment before executing it.

The --no-charset Sass flag makes the CSS filter step of the build process obsolete, but I'm leaving it in for now.