I've never heard of the Delphi IDE "using" an external editor. The integration between the editor and the rest of the IDE (designer, object inspector, tool palette, etc) would likely need a heavy refactor or rewrite. But as @Lars Fosdal mentioned, you can edit Pascal source in any text editor. The project below is a Delphi plug-in that adds a command to the Tools menu to open the current unit in Visual Code. With a little configuration, you could switch between Delphi and Visual Code at will, taking advantage of features in Visual Code such as Copilot.
EditInVsCodeDelphiPlugin