Fix deprecated SCSS syntax, separate LibSass from Dart Sass

[paulthewalton]

• switch from @import to Sass modules (@use "main")
• switch to Sass built-in modules (@use "sass:math", etc...)
• switch from if() SCSS syntax to @if exclusively (deprecated due to collision with native CSS if())
• bumped versions of node-sass (LibSass) and sass (Dart Sass) to latest
• switched from Dart Sass legacy JS API to modern JS API in tests
• separated out LibSass source and tests from Dart Sass due to diverging syntax
• moved "Enable Class" base styles out of _rfs-media-query-rule mixin since Dart Sass no longer hoists same-selector declarations (for more predictable behaviour with modern native CSS nesting)
• added fix for mixed-decls behind a flag so that default output behaviour is unchanged

To enable the fix for mixed-decls, load the RFS SASS/SCSS module with

@use "<path/to/rfs>/scss" with (
    $rfs-no-mixed-decls: true
);

[paulthewalton]

Should fix #363 , #474 , #492