Add a "starlark" renderer for builtin types

Summary:
In order to show users something a bit more friendly for documentation of builtins, it makes sense to render them like the code they work on day-to-day.

So add something that takes Doc objects and spits out a pseudo starlark file that can be parsed / returned by the LSP to the client

Reviewed By: ndmitchell

Differential Revision: D38307725

fbshipit-source-id: e8e093e78a998791abb99693fce7a210dd1aed55

Author: Natalie Jameson