Add code generation for doubly linked list

codegen/dbl_list/snippets/list_insert

@@ -0,0 +1,29 @@
+  if (!list || !node || !(node->string)) {{
+    return;
+  }}
+
+  if (index == 0) {{
+    wapp_str8_list_push_front(list, node);
+    return;
+  }} else if (index == list->node_count) {{
+    wapp_str8_list_push_back(list, node);
+    return;
+  }}
+
+  {T}Node *dst_node = wapp_str8_list_get(list, index);
+  if (!dst_node) {{
+    return;
+  }}
+
+  {T}List node_list = {Tlower}_node_to_list(node);
+
+  list->total_size += node_list.total_size;
+  list->node_count += node_list.node_count;
+
+  {T}Node *prev = dst_node->prev;
+
+  dst_node->prev = node_list.last;
+  prev->next     = node_list.first;
+
+  node_list.first->prev = prev;
+  node_list.last->next  = dst_node;