Maybes in Interfaces should be optional

[notquiteamonad]

Currently, given an elm record type:

type alias User =
  { uid: String
  , email: Maybe String
  }

the resultant DTS will be

export interface User {
  uid: string;
  email: string | void | null;
}

This doesn't actually allow the parameters to be omitted, as the void was intended to do. Instead, the following DTS should be generated:

export interface User {
  uid: string;
  email?: string | null;
}