Return an Optional annotated type predicate function that returns true if the type of x is T or undefined.

To enhance performance, users are advised to cache the return value of this function and mitigate the creation cost.

import { is } from "@core/unknownutil";

const isMyType = is.OptionalOf(is.String);
const a: unknown = "a";
if (isMyType(a)) {
  // a is narrowed to string | undefined
  const _: string | undefined = a;
}