Figure out how to consume design tokens in client-side JS

[emersonthis]

Related to: #718

This issue was briefly discussed in Slack in connection with implementing different behavior for the Sky Nav at different breakpoints...

Background
Our design tokens live in centralized yaml files that are consumed by The Patterns. At the time of writing this, we are importing them into scss files. But we do not (yet) reference them directly in client-facing JavaScripts.

The tool we are currently using for abstracting tokens is Theo. However, Tyler expressed interest in this alternative as well: https://amzn.github.io/style-dictionary/#/

Def of Done

• Ability to reference a design token variable in a client-facing JavaScript file
• a solution for how to use em values in a JavascriptContext*

* This may be trickier than it sounds, because we're currently using fluid font sizes that scale up/down proportionally to the viewport width. So it's not as simple as multiplying by the font-size (integer) of the relevant breakpoint

[tylersticka]

I'm about to submit a PR for the first "def of done."

I've removed the second point because (a) Theo has format transformations built in and (b) I don't think we'll actually need to use them.