feat(defaults): store spellfile in stdpath('data') #33048

Problem:
First rtp directory is unpredictable and not in line with XDG
base spec.

Solution:
Use stdpath('data')/spell as directory if 'spellfile' is not set.

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

test/old/testdir/test_spellfile.vim

@@ -1123,6 +1123,7 @@ endfunc
 " When 'spellfile' is not set, adding a new good word will automatically set
 " the 'spellfile'
 func Test_init_spellfile()
+  throw 'Skipped: Nvim defaults spellfile to stdpath("data")/spell/'
   let save_rtp = &rtp
   let save_encoding = &encoding
   call mkdir('Xrtp/spell', 'pR')