Create new themes-based Figma libraries

[pseys]

Duplicate the Salt (Next) Figma libraries and change the variable modes in order to support themes instead of style options. Initially 2 themes should be supported UITK Legacy and Salt 'Enterprise'. A third option may be included to provide a space for testing. The following files need to be duplicated:

• Salt (Next) Components and Patterns
• Salt (Next) Style Library
• Salt Icons
• Salt Colors

all duplicate files must be marked as UNSTABLE until ready for consumer use.

[pseys]

The Salt Theme style library has been updated with all 'legacy' styles. @ivan-calderon and I are roughly 3/4 of the way through remapping the components in the new components and patterns library. We're on track for completion by end of Red Eye.