import { backupext } from "https://deno.land/x/denops_std@v3.10.2/option/mod.ts";
String which is appended to a file name to make the name of the backup file. The default is quite unusual, because this avoids accidentally overwriting existing files with a backup file. You might prefer using ".bak", but make sure that you don't have files with ".bak" that you want to keep. Only normal file name characters can be used, "/*?[|<>" are illegal.