Add TypeScript declaration file.

[jamescdavis]

This adds a TypeScript type declarations file based on https://github.com/emberjs/ember-test-helpers/blob/master/API.md

[Turbo87]

I'm somewhat hesitant of maintaining this in the repo itself unless we use TypeScript for everything directly 🤔

@jamescdavis would you be willing to help port this project to TS if we decided to do so?

[jamescdavis]

@Turbo87 I certainly understand that concern. Yes, I would definitely be willing to help port to TS. That would be the ideal.

[JonRossway]

Is it possible there's a missing interface for WaitUntilOptions?