Validate if flags has unknown attributes.

import { Denops } from "../mod.ts";
import { parse, validateFlags } from "./mod.ts";

export async function main(denops: Denops): Promise<void> {
  const args = [
    "++enc=sjis",
    "++ff=dos",
    "-f",
    "--foo=foo",
    "--bar=bar",
    "--bar=baz",
    "hello",
    "world",
  ];
  const [_opts, flags, _residues] = parse(args);

  validateFlags(flags, ["f", "foo", "bar"]);
}