Prepare 2.0 and fix 1.x for node-sass

[meduzen]

The recently published 1.2.3 is actually not working for node-sass users and require to support the latest SASS spec (typically: using Dart SASS).

As such, it’s time for this package to have:

• 1.x to stick to node-sass (adapt the recent fix for it);
• 2.x to become the new version;
• documentation reflecting the installation specificities, like in easings.scss.