Well -- not exactly how someone would imagine. You can build your GUI in any iOS, macOS, etc application within Xcode and add your view as a target of another platform. It's maybe not the best solution, but one that works :)
Would be wonderful if you can document it in the README, I think a lot of people would love to be able to use the full power of Xcode.
I've found that SwiftUI is reasonably viable to use even without the GUI builder. I find myself hiding it most of the time, since the code usually tells me what I need.