refactor(highlight): make enum of builtin highlights start with 1

This makes it possible to use HLF_ values directly as highlight id:s
and avoids +1 adjustments especially around messages.

src/nvim/eval/funcs.c

@@ -1342,7 +1342,7 @@ static void f_diff_hlID(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
       hlID = HLF_CHD;  // Changed line.
     }
   }
-  rettv->vval.v_number = hlID == (hlf_T)0 ? 0 : (hlID + 1);
+  rettv->vval.v_number = hlID;
 }
 
 /// "empty({expr})" function