add support for the terminal bell

[jguzman-tech]

This code adds ding calls for the terminal bell. By default this will be the audible bell that I was looking for when I made issue #580. The bell is useful in terminal emulators, because some applications use it as an indicator (such as when bash completion fails, or when you've reached the end of a file in vim).

A user could effectively disable ding calls in vterm by putting this in their init file:

; disable dings (in vterm mode only)
(defun disable-vterm-bell ()
  (setq-local ring-bell-function (lambda () nil)))
(add-hook 'vterm-mode-hook 'disable-vterm-bell)

The key was setting the bell function pointer of the VTermScreenCallbacks struct provided by vterm.h.