feat(button): Update Button component to use kind and emphasis parameters

[janiekyu]

Updated button variations to match Figma designs: https://www.figma.com/file/wB3zWdnIvhflPwlJ9lvVr9/Button?node-id=59%3A6742&mode=dev

Breaking changes:

• Changed mode prop to emphasis prop
• Added a kind prop
• Renamed & added new CSS classes

Preview changes:

• Core: https://dev--designwithcompassion.netlify.app/storybook/core/?path=/docs/components-button--docs
• React: https://dev--designwithcompassion.netlify.app/storybook/react/?path=/docs/components-button--docs

[janiekyu]

Closing this PR so I can fix the snapshots.