feat: Add fallback node for custom placement

lua/neogen/generators/default.lua

@@ -75,7 +75,12 @@ return function(parent, data, template, required_type)
 
     if append.position == "after" then
         local child_node = nodes:matching_child_nodes(parent, append.child_name)[1]
-        if child_node ~= nil then
+        if not child_node and append.fallback then
+            local fallback = nodes:matching_child_nodes(parent, append.fallback)[1]
+            if fallback then
+                row_to_place, col_to_place, _, _ = fallback:range()
+            end
+        else
             row_to_place, col_to_place, _, _ = child_node:range()
         end
     end