This may sound like a facetious reply (its not intended as such!), because what I'm suggesting is itself a self-programmable (from the outside-to-in, as opposed

This may sound like a facetious reply (its not intended as such!), because what I'm suggesting is itself a self-programmable (from the outside-to-in, as opposed to emacs/vim from the inside-to-out) almost "live" environment, akin to emacs, but ... have you looked at [acme](https://research.swtch.com/acme https://research.swtch.com/acme)? It does EXACTLY what you describe above (but the rest of the environment is different enough that you need to spend some time to buy into its philosophies)