Rewrite Box component with TypeScript to establish pattern for core components written in TS

[craigpalermo]

Expected tasks:

• Rewrite Box in TypeScript, get tests working
• Configure Babel to handle TS files
• Configure ESLint to handle TS files
• Configure Storybook to handle TS files
• Generate type declaration files for TS components using TSC
• Confirm that addon-docs still works in Storybook
• Figure out why styled-components prop types not working
• Static type checking in ESLint turns out this isn't actually a feature of typescript-eslint
• Figure out how to combine auto-generated type declarations with hand-written declarations in types package
• Prevent Box and BoxProps from being exported by hand-written declarations