Update Figma import script + move generated code to separate files

[mkrause]

This PR refactors the scripts/import.ts script:

• Add a new command to import primitive color values
• Automatically write the generated code to files in the new src/styles/generated directory (rather than the user having to do it manually)
• Also generate TypeScript files to read the tokens if necessary (e.g. to support the Colors.mdx Storybook documentation)

TODO:

• Give the same treatment to the other token types: font sizes, border radii, and sizings
• Update the components that are using tokens that are now missing in Figma.