We used GeSHi for a good while in the past and it's just become too outdated. First, it's server-side only and PHP only, so you're limited in that regard. Second, there is no text highlighted input as it's only meant to be used for display purposes, not input. And finally, it's really starting to show its age and has fallen far behind the competition.
That's okay. I don't think you're prepared for the amount of time and effort it would take over the course of many years to maintain such a thing. Any time a new function is added to any programming language you'd have to push out an update. That's far too much maintenance for the value it provides.
I'm not aware of any online code editors that do this. The top 2 right now are Ace and CodeMirror (which is what we use). Honestly, I don't think this is worth the maintenance it would create. If you look at major code sites like Github and StackOverflow, they don't do it nor do I think it would add value to their service. It's easy enough just to search Google.