Rewrite in TypeScript

[mjwwit]

Rewriting the project in TypeScript will allow automatic generation of type definitions, easing implementation efforts in dependent projects. It can also help improve adherence to the XHR spec.

• Introduce TypeScript as a dev-dependency
• Create a strict TypeScript config
• Rewrite module code into TypeScript and make it pass the compiler checks
• Setup build config that outputs JavaScript and type definitions
• Update package.json to refer to new build artifact locations