My extension, Hacker News Enhancement Suite [0] does this along with other things. Unlike other extensions, I chose to place the collapse icon before the username so it's in a predictable place.

The extension also changes the entire style of HN so some people may not like that. I am working on creating an option to use the classic style.

[0] https://chrome.google.com/webstore/detail/hacker-news-enhanc...