Format key and value to construct string array.

import type { Entrypoint } from "https://deno.land/x/denops_std@v6.5.0/mod.ts";
import { formatOpt } from "https://deno.land/x/denops_std@v6.5.0/argument/mod.ts";

export const main: Entrypoint = async (denops) => {
  console.log(formatOpt("enc", "sjis"));
  // "++enc=sjis"
}