build-sass: Use new Shared Resources API

[aduth]

🛠 Summary of changes

Enhances the @18f/identity-build-sass Sass compilation tool to use the new Shared Resources API added to Sass in a recent version. This feature is well-suited for our use-case, where we compile many different stylesheets, and each stylesheet shares a number of common resources (USWDS, etc.).

Performance:

Average of yarn build:css on a set of 10 runs:

Before: 8.75s
After: 6.71s 2.98s after 3fe4d75
Diff: -2.04s (-23.3%) -5.77s (-65.9%) after 3fe4d75

📜 Testing Plan

Verify that yarn build:css successfully outputs to app/assets/build

[aduth]

I can't recall if I had specifically dug into the performance characteristics of the synchronous vs. asynchronous compilers, but a brief test with initAsyncCompiler shows that it performs quite well, about 3.5s on average. I might look into seeing if it's easy enough to swap to that.

[aduth]

Switched to async compiler in 3fe4d75 and updated performance result in original comment.