Add TypeScript type definitions

[sgmccli]

This pull request provides type definitions for the parent-package-json module. This allows it to be easily integrated with the TypeScript programming language.

[maxrimue]

Thanks for the pr. I appreciate you putting time into this, however, I don't see much value in adding definitions to little projects like this one. Sindre Sorhus, who I share my opinion with, put it like this:

I use neither TypeScript nor Flow in my small modules as I don't yet see enough value. If I were to
accept type definitions, I would have to maintain them. Which I would do badly. I would say bad
definitions are worse than none. DefinitelyTyped and flow-typed are good solutions to this problem.
Submit the type definitions there instead.

See the comment here

[sgmccli]

Ok fair enough. I have submitted a PR on DefinitelyTyped.

[maxrimue]

Thanks!