feat: add Sass implementation

[zvuc]

Adds an official sass port of Tuesday.

Funnily enough this was the very first issue posted as soon as Tuesday went public (#1), we didn't give much attention because we weren't using Sass that much internally at Shakr. Well it's 2020 now and yes we still do use Less for our main code but we started using Sass for other side projects so I decided why not try converting keyframes to Sass.

Based on node-sass compatible syntax (Hence it uses @import instead of the newer @use) due to low adoption rate of dart-sass as of June 2020 (https://www.npmtrends.com/dart-sass-vs-node-sass)

Sass version will only support keyframes imports through @include and is not used for main CSS build for distribution.

After merge, I'm planning to deprecate npm package "tuesday.less" in favor of "tuesday.css" (because now the repo contains source files both in less and sass) https://www.npmjs.com/package/tuesday.less