Feature/simplify and add word colors

[thinkjrs]

This adds basic color extensibility to the button for clients.

Now users can specify word colors when rendering the PromoButton as a prop, e.g.:

<PromoButton
  logoSrc=""
  words={['Super', 'Easy', 'Ads']}
  wordColors={['red', 'slate', 'blue']}
  shape="square"
  email="j.doe@example.com"
  backend="/api/promo"
/>

Current color choices include

• "gray",
• "slate",
• "red",
• "indigo",
• "blue",
• "green",
• "yellow",
• "orange",
• "violet"

[github-actions]

size-limit report 📦

Path	Size
dist/promo-button.cjs.production.min.js	74.46 KB (+0.38% 🔺)
dist/promo-button.esm.js	58.9 KB (+0.52% 🔺)