vim-patch:9.0.0138: not enough characters accepted for 'spellfile'

Problem:    Not enough characters accepted for 'spellfile'.
Solution:   Add vim_is_fname_char() and use it for 'spellfile'.

bc49c5f48f

Cherry-pick related doc update from Vim runtime.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/spell.c

@@ -3621,7 +3621,7 @@ bool valid_spellfile(const char *val)
   FUNC_ATTR_NONNULL_ALL FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT
 {
   for (const char *s = val; *s != NUL; s++) {
-    if (!vim_isfilec((uint8_t)(*s)) && *s != ',' && *s != ' ') {
+    if (!vim_is_fname_char((uint8_t)(*s))) {
       return false;
     }
   }