I fully agree with the idea of allowing users to write code outside of the game and I think that it is a worthy investment even if a game requires the user to write code (maybe especially then). However, I also think a basic editor right into the game isn’t a bad thing to have.
When I still played Minecraft with the computercraft mod I used to basically write everything using an external editor because the basic editor available is shitty. However, I lost count at how many times being able to quickly edit something made my life a 1000 times easier.
Luckily an in game editor in that scenario doesn’t need to be that complicated. Just being able to view at what line I’m at was good enough for pretty much all those times (so, no code highlighting and no auto complete)
There is one thing to consider though: how likely is it that your target audience has 2 or more monitors? Based on that answer you may need to think about ways to make people comfortable switching to another program (thus not being able to see what is going on in the game) or make the in game editor good enough to where people feel they can safely use it without missing out on too many features they would otherwise have.