Add type definitions

[rohit-gohri]

Helpful links:

• https://github.com/Microsoft/TypeScript/wiki/JsDoc-support-in-JavaScript
• https://www.typescriptlang.org/docs/handbook/type-checking-javascript-files.html
• https://www.typescriptlang.org/docs/handbook/declaration-files/do-s-and-don-ts.html
• https://github.com/Microsoft/vscode-docs/blob/master/docs/languages/javascript.md