Improve TypeScript typings

[fahad19]

Typings in all the packages can be improved further.

Someone who is into TypeScript may want to take a look at the packages in the monorepo :)

For local development guide, please see: https://github.com/fahad19/proppy/blob/master/MAINTAINERS.md

[caseyWebb]

I'll take a pass on the repo. Can I swap tslint for eslint+@typescript-eslint and introduce a formatter (prettier) while I'm at it?

[fahad19]

@caseyWebb: sure thing!