Add TypeScript definitions

[mhink]

Based on this conversation:

https://twitter.com/mhink1103/status/854064651495133184

It looks like we'll need to add and package a .d.ts file which overrides TypeScript's JSX.IntrinsicElements interface. It looks like they have something called __React.HTMLAttributes involved. Reference the following links for more details:

microsoft/TypeScript#4653
microsoft/TypeScript#4648