Ensure the executor is executed under a modifiable buffer

import type { Entrypoint } from "https://deno.land/x/denops_std@v6.5.0/mod.ts";
import * as fn from "https://deno.land/x/denops_std@v6.5.0/function/mod.ts";
import { modifiable, open } from "https://deno.land/x/denops_std@v6.5.0/buffer/mod.ts";

export const main: Entrypoint = async (denops) => {
  await open(denops, "README.md");
  const bufnr = await fn.bufnr(denops);
  // ...
  await modifiable(denops, bufnr, async () => {
    await fn.setline(denops, 1, ["Hello", "World"]);
  });
}