refactor(options): remove obsolete distinction of "vi" vs "vim" defaults

It might come as a schocking surprise, but the defaults we support
are the NEOVIM defaults.

src/nvim/testdir/setup.vim

@@ -20,7 +20,7 @@ set tags=./tags,tags
 set undodir^=.
 set wildoptions=
 set startofline
-set sessionoptions&vi
+set sessionoptions+=options
 
 " Prevent Nvim log from writing to stderr.
 let $NVIM_LOG_FILE = exists($NVIM_LOG_FILE) ? $NVIM_LOG_FILE : 'Xnvim.log'