Provide flow types / libdefs

[rgbkrk]

For many of our functions type annotations with well specified objects would be super helpful for end users to know what options are available.

• [x] Include the babel transform to strip flow types
• [ ] Provide flow types for each api
  • [ ] contents
  • [ ] kernels
  • [ ] kernelspecs
  • [ ] sessions
  • [ ] terminals
• [ ] Export flow types in a way that can be picked up by a dependent package

[jdetle]

Thanks for this Kyle! I'll start working on it soon. (Update: clearly I need to stop saying I'll do things when I don't get around to them)

[captainsafia]

First checkbox on this has been completed and the second has been partially completed. More flow typing to come.