Nvim: use XDG data directory

Closes https://github.com/mhinz/vim-startify/pull/351

autoload/startify.vim

@@ -14,7 +14,8 @@ let s:numfiles       = get(g:, 'startify_files_number', 10)
 let s:show_special   = get(g:, 'startify_enable_special', 1)
 let s:relative_path  = get(g:, 'startify_relative_path') ? ':~:.' : ':p:~'
 let s:session_dir    = resolve(expand(get(g:, 'startify_session_dir',
-      \ has('win32') ? '$HOME\vimfiles\session' : '~/.vim/session')))
+      \ has('win32') ? '$HOME\vimfiles\session' :
+      \ has('nvim') ? stdpath('data') . '/session' : '~/.vim/session')))
 let s:tf             = exists('g:startify_transformations')
 
 let s:skiplist = get(g:, 'startify_skiplist', [

doc/startify.txt

@@ -134,7 +134,8 @@ default values.
 <
 The directory to save/load sessions to/from.
 
-The default for Windows systems is '$HOME\vimfiles\session'.
+The default for Windows systems is '$HOME\vimfiles\session'. The
+default for Neovim is '$XDG_DATA_HOME/nvim/session'.
 
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                                                          *g:startify_list_order*