Add code generation for doubly linked list

codegen/dbl_list/snippets/list_get

@@ -0,0 +1,13 @@
+  if (index >= list->node_count) {{
+    return NULL;
+  }}
+
+  {T}Node *output  = NULL;
+  {T}Node *current = list->first;
+  for (u64 i = 1; i <= index; ++i) {{
+    current = current->next;
+  }}
+
+  output = current;
+
+  return output;