I'm looking for an editor/basic IDE I can use to build an IDE for a programming language
on.
I doubt it's a good idea to build an entire IDE on your own - especially in 2017. There are so many great and extensible IDEs these days. Why do you want to waste your time with upcoming bugs and feature requests like "Search all files doesn't work wih regular expressions" or "Files with chinese characters don't show up properly"? There are tons of features users need to have implemented before they consider an IDE as good.
If you want to create a programming language you might want to focus on the important: A compiler/interpreter, syntax highlighter and good language support in the editor like code completion and quick fixes. For great language support you can implement a language server based on the
language server protocol that can be easily plugged into existing
language server clients such as Visual Studio or Eclipse on the IDE side and Visual Studio Code, Atom or emacs on the text editor side.
Good IDEs and text editors are the result of an at least medium sized team spending tons of days on the product. You can profit from their work.