Add code generation for doubly linked list

codegen/dbl_list/snippets/list_empty

@@ -0,0 +1,8 @@
+  if (!list) {{
+    return;
+  }}
+
+  u64 count = list->node_count;
+  for (u64 i = 0; i < count; ++i) {{
+    wapp_str8_list_pop_back(list);
+  }}