Add oneshot anonymous functions as denops API and return the identifiers

import { Denops } from "../mod.ts";
import * as anonymous from "./mod.ts";

export async function main(denops: Denops): Promise<void> {
  // Add anonymous functions
  const ids = anonymous.once(
    denops,
    () => {
      // Do what ever you want.
    },
    () => {
      // Do what ever you want.
    },
    // You can add as many callbacks as you want...
  );

  // First calls complete normally
  await denops.dispatch(denops.name, ids[0]);
  await denops.dispatch(denops.name, ids[1]);

  // But second call would throw errors
  await denops.dispatch(denops.name, ids[0]);
}