Migrate compiler to use Dart Sass

[apalfrey]

Since 2020-10-26, node-sass has been deprecated: https://sass-lang.com/blog/libsass-is-deprecated

Currently this project is using gulp-sass for it's compiling which uses node-sass. While gulp-sass DOES allow you to pick a compiler to use, it still includes node-sass as a dependency which is unnecessary.

gulp-dart-sass is a fork of gulp-sass which already uses Dart Sass (as it's name suggests). However, due to the way Yarn works, we can only work with the version of Dart Sass it supplies. While this is currently the latest version, including any the Bootstrap variables file (required for our variables) results in deprecation warnings for division operators: twbs/bootstrap#34051

So, this will have to be done in steps in order to ensure compatibility remains without warning messages;

• Replace Yarn with vanilla npm
• Replace gulp-sass with gulp-dart-sass
• Add sass@1.32.13 to prevent deprecation messages
• Update sass to latest once Bootstrap updates with deprecation fixes

The first 3 parts should be fine to do straight away, however the final part will require waiting until the fixes are in place on Bootstraps end. Once they are, if any issues crop up from this project, we can take care of them as necessary