Add TypeScript typings for global helpers

[chriskrycho]

I know, I know: sort of churn-heavy given the goal is for the unified test helpers to land. But, in the mean-time… my files all look like this:

Note: I'm fully planning to implement these for our use, and will contribute them to resolve this issue once I have them all available. I'm mostly making this issue as a reminder for me and to make it easy for others to contribute if they already have them. 😛

[chriskrycho]

Oh, hey, these actually come in from Ember itself (which is a huge surprise to me!).