You can use any game engine you like, but you must upload a digital game to itch.io, which is playable by someone using a Windows PC, with nothing more than a keyboard or mouse. It can be a download or a browser-based game (or both!) It should not require additional hardware or software to run.
A browser extension is browser-based and fits within those rules.
However, it could be argued that this entry does not belong in GMTK Jam as it isn’t a game. But while the whole rules are written based on the assumption that submissions are games, it is never stated that you can’t submit something that isn’t a game.