[Feature request] Send multiple input events to libvterm through `vterm-send-next-key`

[heiwiper]

Enable vterm-send-next-key to accept a universal argument, allowing multiple keystrokes to be sent at once (e.g., C‑u 3 M‑x vterm‑send‑next‑key would forward the next three keys to the terminal). This would simplify using external editors like Nano or Jmacs on remote servers where TRAMP isn’t feasible.