I am a professional software developer and i really love the concept of the game. But the in-game code editor is quite painful to use for me, since i am used to the features of IDEs that have been developed and improved over many years.

Instead of adding support for using external editors, a simpler first step would be to provide the interfaces/signatures of all the available functions and tables/classes/objects via LUA scripts.
This way I could import the interfaces/signatures, write my code with IntelliSense support in any editor of my choice, then copy it to the in-game editor and run it.

My experience with LUA is limited, so i'm not sure if this interfaces/signatures can be generated automatically from the existing LUA scripts used by the game. Creating them manually is probably a lot of work, especially when they need to be maintained with every update.

Another option would be to provide the LUA scripts the game is using internally, so the user can import them. Maybe not publicly, but as part of the game files installed on the system?