Does the new IDE prevent the ability to use an external editor now? The option is missing from the preferences. I assumed that in order to handle intellisense-like features, maybe the IDE has to use only an internal editor. Is that the case?
You are welcome to edit your code in an external editor. Arduino IDE 2.x uses a more standard and intuitive approach to this. Just open the sketch up in an external editor and edit away. The changes will immediately appear in the IDE's editor.
There is no need to put it in any sort of special mode and so there is no need for a confusing "Use external editor" preference item.
Just make sure that the sketch open in the Arduino IDE is in a saved state. It will not reflect the external changes if it is in a "dirty" unsaved state. But that won't be a problem if you have the "Auto save" preference enabled.