feat: improve dropped question support

src/export/practice.rs

@@ -602,6 +602,7 @@ items:
                     learning_objectives: Vec::new(),
                     credit_overrides: Default::default(),
                     dropped: false,
+                    dropped_as: None,
                 },
                 Placement {
                     number: 2,
@@ -615,6 +616,7 @@ items:
                     learning_objectives: Vec::new(),
                     credit_overrides: Default::default(),
                     dropped: false,
+                    dropped_as: None,
                 },
             ],
         }