Ensure the executor is executed under a modifiable buffer

import { Denops } from "../mod.ts";
import * as fn from "../function/mod.ts";
import { modifiable, open } from "../buffer/mod.ts";

export async function main(denops: Denops): Promise<void> {
  await open(denops, "README.md");
  const bufnr = (await fn.bufnr(denops)) as number;
  // ...
  await modifiable(denops, bufnr, async () => {
    await fn.setline(denops, 1, ["Hello", "World"]);
  });
}