feat(DesignTokens): nest default and dark colors in same tokens files

Add `default` and `dark` properties to `$value` property in color design
tokens with dark mode variants
Remove `.dark.tokens.json` files
Add style-dictionary links to comments in `build_tokens.js`
Simplify src and dist directory config to static strings
Use uppercase for PREFIX constant
Update to use lodash merge on style-dictionary config
Update condition to check truthiness of token.original.value