feat(Tokens): add theme color design tokens

Add storybook stories for color palettes
Move data-viz color tokens to own `.tokens.json` file
Add snapshots for stories
Replace `{gray.50}` dark mode white value with hex value for simplicity
Add dark color stories
replace duplicated `template` definitions with shared constant
Add `contrastRatio` argument to `colorFromBackground` util function
Use `colorFromBackground` to compute text color for 4.5:1 ratio
Remove $theme- SCSS variables