Type predictions for union types

[EnixCoda]

This PR provides prediction functions that distinguish union types, which are frequently used in request return values. It works very well in my own use cases.

Usage example (modified examples/database-update-send-email):

Should help resolve #92

PS: The added file would look better if printWidth of prettier could be set to larger number than the default 80 :P

[CLAassistant]

All committers have signed the CLA.

[justjake]

This is a nice idea, but closing as we're planning to replace many of the existing hand-authored types with OpenAPI-generated types.

[EnixCoda]

So you will deprecate the union types since generating the types from OpenAPI? If not, I am very curious how would you resolve union types. Looking forward to your progress.

[justjake]

@EnixCoda The types may change a bit as we do the transition, and using an OpenAPI schema may allow us to generate type guard functions as well. For those reasons it doesn’t make sense to accept this changeset at the moment. We can revisit after the new types land.