[#693] Export design tokens to JS

[planktonic]

Fixes #693

Changes

• Adds export files for all the design tokens
• Adds documentation for using the design tokens in JS

Example

import breakpoints from '../exports/_breakpoints.scss';

console.log(breakpoints['s']); // 'screen and (max-width:29.9375em)'
console.log(breakpoints['m']); // 'screen and (min-width:30em)'
console.log(breakpoints['l']); // 'screen and (min-width:55em)'
// …

import colorPalette from '../exports/_color-palette.scss';

console.log(colorPalette['brand-1.lighter']); // '#f4f2ff'
console.log(colorPalette['brand-1.light']); // '#c8bfff'
console.log(colorPalette['brand-1.base']); // '#644dff'
// …

Checks

Delete if not applicable:

• [x] Storybook documentation has been updated
• [x] Your changes have been added to the unreleased section of CHANGELOG.md