Support Paragon design tokens and theming

[arbrandes]

Provide all the features necessary for design tokens to work, as currently implemented in Paragon, frontend-build, and frontend-platform.

Porting notes

• We don't need to load base paragon styles, so all related hoops that are currently jumped through in master are not needed.
• Usage notes on MFE design-token branches (to clarify how the feature works)
• There's a command (paragon replace-variables) to replace SASS with CSS vars automatically, when converting MFEs