Avoid E151 when generating helptags without Vim help files

[Copilot]

dpp#make_state() could invoke :helptags for a non-empty documentation directory containing no .txt files, causing Neovim error E151.

• Detection: Recursively detect actual .txt help files before invoking :helptags.
• Regression coverage: Verify empty directories and non-help files are skipped, while nested help files are detected.

if (await hasHelpFiles(docDir)) {
  await denops.cmd(`silent helptags ${docDir}`);
}