Am I forced to use Windows programming (win32) here?
Yes. The *nix world can't have text editors :( It's very sad. We have to feed binary directory into files and pray it works.
All jokes aside, you can use whatever you'd like. It is quite the project though. My suggestion is go look at various other text editors and then see what features you like/dislike and get a good idea of what your final product will do.