Read a mapping and return a Mapping instance.

import type { Denops } from "https://deno.land/x/denops_std@v6.3.0/mod.ts";
import * as mapping from "https://deno.land/x/denops_std@v6.3.0/mapping/mod.ts";

export async function main(denops: Denops): Promise<void> {
  await denops.cmd(`map <Plug>(test-denops-std) Hello`);

  console.log(await mapping.read(denops, "<Plug>(test-denops-std)"));
  // {
  //   mode: "",
  //   lhs: "<Plug>(test-denops-std)",
  //   rhs: "Hello",
  //   // ...
  // }
}

Note that prior to Vim 8.2.0491 or Neovim 0.5.0, script is not detectable by maparg function internally used in this function.