Add SASS unit test framework with sass-true

[jesstelford]

Depends on #10489

The SASS functions are starting to get more complicated, yet are untested. Given their foundational use within Polaris, @gwyneplaine & I figured it was time we added some unit tests.

This PR sets up SASS True for running unit tests on SASS within Jest, and includes some sample tests to begin with.

[sam-b-rose]

Definitely in favor of making our tests more robust to support and validate advanced styling functions. However, I think the direction is to remove SCSS from web. Polaris will likely follow

Is there a way we could support features and tests like this using PostCSS or native CSS? Or perhaps we adopt a different strategy for handling responsive props down the road. This example is not a 1:1 "replacement", but maybe we go back to first-principles on this and approach it from a different angle.

[jesstelford]

I think the direction is to remove SCSS from web. Polaris will likely follow

Yep, am keeping that at the front my mind while I work on these 👍 I outlined here a couple paths forward after this work is merged:

Depends on what we're aiming for;

1. If we're going to PostCSS, then most of the functionality can be moved into postcss-mixins by the looks of it.
2. If we're going to CSS-in-JS, then the functionality can be converted to JS fairly easily.
3. If we're going to pure CSS without PostCSS plugins or CSS-in-JS then we could either fallback to creating and injecting stylesheets, or reassess the overall strategy.

[jesstelford]

Superseded by #11252