Connect legacy palette variables to characteristics in Figma

[pseys]

Connect legacy palette variables to characteristics within the Salt Next Styles Library.

• [x] #4113
• [ ] Map new variable collections to incorporate 'legacy' colors/palette and typography
• [ ] Map legacy mode characteristics to legacy palette variables based on existing code
• [ ] Publish updated library

Considerations

• The characteristic variables will need modes applied; Salt Next and Salt Legacy (TBC)
• Each characteristic variable mode will need to be mapped to a different set of palette variables – the Salt Next characteristics are already aligned to Accent / Mode in the styles library. Salt Legacy will need to map to a 'Mode' only switch, either in the same file or from a different library (TBC)
• When creating a new design, components will default to Salt Next, but designers will have the ability to toggle to Salt Legacy if they wish, however, they will have to set mode separately

Dependent on #3843 and #3844