Add 3D printing home role with orca-slicer and openscad

Create a new home-manager role for 3D printing applications including
orca-slicer for slicing and openscad-unstable for 3D modeling. Enable
the role in the desktop configuration for full-featured experience.

home/roles/default.nix

@@ -3,6 +3,7 @@
   # Platform-specific roles are imported via base-linux or base-darwin
   # in each home configuration file
   imports = [
+    ./3d-printing
     ./base
     ./communication
     ./desktop