Initial project setup for new Stitches based design system components

• New Rush project for the new design system components. I've chosen to go with a seperate project rather than making a major change to the existing components so that the (few) projects can continue to patch existing components without being forced into rewriting a chunk of the codebase.
• Uses Storybook as a local dev environment to preview components and their variants
• Uses Stitches as it's styling library, leveraging design tokens.
• Uses a library for demo colour scales. This will be replaced by our own token library once the scales have been designed.
• Uses Rollup to bundle
• Currently does not publish to npm
• No tests as the example components are simply styles, but the intention is the use react-testing-library when more complex components are built

The component styles are illustrative at the moment, so can be left out of the review process.