Return a type predicate function that returns true if the type of x is a literal type of pred.

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.LiteralOf("hello");
const a: unknown = "hello";
if (isMyType(a)) {
  // a is narrowed to "hello"
  const _: "hello" = a;
}