new: Support single range colors.

[milesj]

Summary

All our colors require 10 shades, even for simple colors like black and white. This change allows colors to either be a range of 10 shades, or a single shade.

# Before
white:
  '00': '#fff'
  '10': '#fff'
  '20': '#fff'
  '30': '#fff'
  '40': '#fff'
  '50': '#fff'
  '60': '#fff'
  '70': '#fff'
  '80': '#fff'
  '90': '#fff'

# After
white: '#fff'

Screenshots

Checklist

• [x] Build passes with yarn run check.
• [x] Code is formatted with yarn run format.
• [x] Tests have been added for my changes.
• [x] Code coverage for my change is 100%.
• [x] Documentation has been updated for my changes.