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/

.config/vim/vimrc.d/40-keys.vim

@@ -11,11 +11,6 @@ nnoremap <silent> <Esc> :nohlsearch<CR><Esc>
 " Create new panes
 nnoremap <C-w>N :vsplit<CR>
 nnoremap <C-w>n :split<CR>
-" Move between panes
-nnoremap <C-h> <C-w><C-h>
-nnoremap <C-j> <C-w><C-j>
-nnoremap <C-k> <C-w><C-k>
-nnoremap <C-l> <C-w><C-l>
 
 " Substitute command
 if (exists('+inccommand') && &inccommand != '')