Infra: Include `sass` as a peer dependency

[daneah]

The problem

Pharos isn't targeted for full end-user consumption without a build process. In particular, it exposes much of its styling as uncompiled SCSS rather than compiled CSS. As such, consumers should expect to include sass and appropriately configure it for their needs.

Right now, Pharos doesn't communicate this fact and projects can end up in a state where the project builds successfully, but produces improperly styled pages because the SCSS either ends up on the page uncompiled or possibly not at all.

We want systems to complain much more loudly about the lack of a SCSS compilation step.

The solution

Specify sass as a peer dependency

Alternatives considered

We could update Pharos to provide fully-compiled CSS files for everything. This might actually be a reasonable long-term approach, but needs a bit more exploration and rationalization. Adding the peer dependency is an easy (and reversible) stop-gap in the interim.