Cleanup and improve style-guide package

[atimmer]

Explanation

The style-guide package has a bunch of leftover decisions and consequences we need to cleanup and improve:

• We are using the scss_to_json package which is not very well maintained, it's better to use a script we use ourselves co-located with the package.
• We use a SCSS file as the source of truth and the JSON is generated from this. We need to use a JSON/JS file as the source of truth and generate the SCSS file from that. The SCSS file can then be removed if we manage to ever drop our usage of SCSS completely.
• The JSON now has $ in front of the variable names, we should drop these because they are leftovers from a SCSS environment.
• We have two types of colors in one document: palette-colors and usage-colors.

Technical decisions

A draft for a possible setup:

src/
  palette.js
  colors.js
  schemes.js
tools/
  transform.js    (Contains transforms to older `colors.json` and `_colors.scss`)

Feedback?