vim-patch:9.0.1798: The 'syntax' option has no completion. (#24908)

Problem:  The 'syntax' option has no completion.
Solution: Add syntax option completion.

closes: vim/vim#12900

6dfdff3f27

N/A patches:
vim-patch:9.0.1795: Indentation issues

Co-authored-by: Doug Kearns <dougkearns@gmail.com>

src/nvim/option.c

@@ -5394,6 +5394,11 @@ void set_context_in_set_cmd(expand_T *xp, char *arg, int opt_flags)
 
   xp->xp_pattern = p + 1;
 
+  if (options[opt_idx].var == &p_syn) {
+    xp->xp_context = EXPAND_OWNSYNTAX;
+    return;
+  }
+
   if (flags & P_EXPAND) {
     p = options[opt_idx].var;
     if (p == (char *)&p_bdir