Adjust `~/.tmux.conf` contents based on installed tmux version

[jwodder]

• When tmux version is >= 3.6, include:

  set -g visual-bell off  # "both" when < v3.6
  
  set -wg copy-mode-selection-style "reverse"
  set -wg pane-scrollbars off
  
  set-hook -g alert-bell {
      if -F "#{!:#{window_active}}" 'display "Bell in window #{window_index}"'
  }
  

• When tmux version is >= 3.5, include:

  set -wg allow-set-title on
  

• When tmux version is >= 3.4, include:

  set -wg allow-passthrough all  # "on" when < v3.4
  

• In addition to the above, when tmux version is < 3.4 (or 3.5?), the t_RB definition in ~/.vim/vimrc for tmux needs to be changed to:

  if stridx(environ()['TERM'], 'tmux') == 0
      " Sequence for querying background color for setting 'bg':
      let &t_RB="\<ESC>Ptmux;\<ESC>\<ESC>]11;?\<C-G>\<ESC>\\"
  endif