vim-patch:8.2.2728: special key names don't work if 'isident' is cleared

Problem:    Special key names don't work if 'isident' is cleared.
Solution:   Add vim_isNormalIDc() and use it for special key names.
            (closes vim/vim#2389)
e3d1f4c982

Code is N/A as Nvim already has ascii_isident(), so just port the test.

src/nvim/testdir/test_mapping.vim

@@ -433,9 +433,12 @@ func Test_list_mappings()
   " Remove default mappings
   imapclear
 
-  inoremap <C-M> CtrlM
+  " reset 'isident' to check it isn't used
+  set isident=
+  inoremap <C-m> CtrlM
   inoremap <A-S> AltS
   inoremap <S-/> ShiftSlash
+  set isident&
   call assert_equal([
 	\ 'i  <S-/>       * ShiftSlash',
 	\ 'i  <M-S>       * AltS',