feat: make visual bell an option @tnotify-visual-bell, as the default command isn't compatible with *Nushell*

[stevenxxiu]

The option name is from #53 (comment).

Not in the PR, but also on my system, the default command clears the bottom line. I use Ghostty and Nushell. In Nushell, this is required in the script instead (not included in the PR):

tmux split-window -h -t "\$$SESSION_ID":@"$WINDOW_ID" "print \"\\a\"; exit"

To have Nushell show a bell with this PR, turn this off and use a custom command:

set-option -g @tnotify-visual-bell "off"
set-option -g @tnotify-custom-cmd 'tmux split-window -h "print \"\a\""'