I one were to start from scratch, writing an emacs-like text editor by building it on top of a small programming language you can use for customizing and extend

I one were to start from scratch, writing an emacs-like text editor by building it on top of a small programming language you can use for customizing and extending the editor, Lua would a great choice. Lua has only a tiny standard library, but in writing the core of the editor, you probably have to supply the equivalent functionality yourself, anyway. But I do not see how you could bolt Lua on to GNU Emacs. You would have to start from scratch, and that would be a lot of work. And even then, there is the fact, that GNU Emacs is already well-established, has a very loyal user base and is - by and large - good enough, even great for most purposes, so even if you wrote LMacs or whatever one would call it, you would probably end up with, like, a dozen users or so. (IIRC the Zile project, a small emacs-like editor, has tried this, but I do not know how far they got with this.)