This is speculation, but I think there's a way to do it without much pain at all. I've built a few VSCode extensions and the extension framework is extensive and robust and it has well defined boundaries. The only problem you'd run into is the extension sandbox. If all your fork does is make a sandbox exception for your specific code at the extension boundary, you will only have to keep those bits correct and then you can write the whole rest of your custom editor like you would an extension. That would mean that you get to benefit from all the existing know how around building extensions and also make keeping up with upstream a breeze.

As I said though, this is speculative and how I would approach this problem based on the work I've done, I don't know if that's realistically feasible and what they've actually done.