I've never seen a script that does that. You'd need a dictionary file as well as having the script constantly search it every time you type something. With mIRC being single threaded, my guess is that something like that will probably cause a lot of lagging/freezing within mIRC. Perhaps it can be written in a way to avoid that, but more than likely it just won't work well in mIRC.
It's much better to work on good typing skills anyhow, imo.