Use react-aria as base for existing components

[JBurkinshaw]

@atweedie and @JBurkinshaw agreed that going forward, react-aria will act as the base component library for this component library.

A proof of concept has been completed by @JBurkinshaw, there is an open PR (#34) and the associated branch is 30-react-area-poc. The work can be based on this.

Describe the solution you'd like

• For existing components, update them so they are built using react-aria components. This will include at least the following (please add anything that is missing):
  • Layer button reel
  • Layer card
  • Slider control
  • Swatch
  • Swatch legend
  • Icon
• Consider integration with existing theming methods. In the POC PR, there is more scope for using CSS variables.

Describe alternatives you've considered
We discussed the current approach with no base component library. We also discussed Radix, along with a few other alternatives (see #30)