WIP: JSig spec

I started working on a more formal spec.

I've defined the following statements

 - assignment
 - type declaration
 - import statements

I've defined the following Types

 - builtin types
 - custom type literals

cc @Matt-Esch @jden

cc @bryan-m-hughes Hi, we talked at forwardjs about
    typed javascript! I started working on a spec for 
    this kind of stuff, would love some feedback :)