Explicit, framework agnostic, CSS-in-JS support

[saxoncameron]

Currently our CSS-in-JS support relies on user's bootstrapping their own implementation using the resolveBp function export. For official support, we'll need to have a proper review of it in practice with popular CSS-in-JS frameworks: at the very least Emotion and Styled Components.

Ideally this comes down to something like just three functions:

• cq(query, bpName) a.k.a. container query, (also known as bpc) which conditionally returns a CSS block based on the provided query
• mq(query) a.k.a. media query, which returns a legit CSS media query block

For SCSS support, we might need to add a third mixin for the app container. Really there are three use-cases here overall:

• Container query
• App container query (this can be achieved in CSS-in-JS by just passing through the AppBreakpoint bpName)
• Media query

But for SCSS we'll need to add a third mixin targeting the className injected by the AppContainer to support the above.