Emacs basically has this. It is why I mentioned it. There are a set of functions that are compiled c code, but the majority of emacs is just a click away from my looking at it and modifying if I wanted to.

And that is the thing, I rarely want to. Or, I'll want to, but it isn't easy enough to do that I can get it done. Often, the thing I would like to do is a melpa install away. And the amount of code involved with many features is far beyond my skillset, it usually seems.