Previously deprecated in v4.x, this clears out the now unused Sass option and removes some unused mixins. Arguably we could remove more, but I like the hover-focus mixin and we make extensive use of it across the project.
* Migration docs page for v5 * Add JS section, tweak some copy * Update migration.md
This allows for the build to fail if a reference isn't found.
No more Ruby dependency and most important more than 10 times less build time (at least on Windows)!
