Emit [@@js.builder] if the interface is POJO (i.e. no methods)

[cannorin]

interface Person {
  name: string;
  age: number;
  children?: Person[];
}

module Person: sig
  type t
  val create: ~name:string -> ~age:int -> ?children:t list -> t [@@js.builder]
end

Maybe we should add unsafe cast functions too?