Format key and value to construct string array.

import type { Entrypoint } from "https://deno.land/x/denops_std@v6.5.1/mod.ts";
import { formatFlag } from "https://deno.land/x/denops_std@v6.5.1/argument/mod.ts";

export const main: Entrypoint = async (denops) => {
  console.log(formatFlag("f", ""));
  // "-f"

  console.log(formatFlag("foo", "value"));
  // "--foo=value"
}