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

[craigpalermo]

Expected tasks:

• [x] Rewrite Box in TypeScript, get tests working
• [x] Configure Babel to handle TS files
• [x] Configure ESLint to handle TS files
• [x] Configure Storybook to handle TS files
• [x] Generate type declaration files for TS components using TSC
• [x] Confirm that addon-docs still works in Storybook
• [x] 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~