vim,tmux:plugs: Add vim-tmux-navigator

[vim-tmux-navigator][1]:

> Seamless navigation between tmux panes and vim splits

Add the hybrid vim/tmux plugin `vim-tmux-navigator` to switch between
tmux panes and vim splits with `C-[hjkl]`.

Add vim-tmux-navigator as a submodule in tmux/plugins and create a
symlink from vim/pack/plugins/start.

Get rid of similar switching keymappings in the vim and tmux configs.

[1]: https://github.com/christoomey/vim-tmux-navigator

.config/vim/pack/plugins/start/vim-tmux-navigator

@@ -0,0 +1 @@
+../../../../tmux/plugins/vim-tmux-navigator/