fix: Codegens spell items (#1765)

### Summary
- Fixes some serialization issues with spell items
- Fixes accessibility issues with field spells and removes the awkward "InternalItem" concept
- Codegens the fields/items used in spells

Projects/UOContent/Migrations/Server.Spells.Fifth.PoisonField.v0.json

@@ -0,0 +1,19 @@
+{
+  "version": 0,
+  "type": "Server.Spells.Fifth.PoisonField",
+  "properties": [
+    {
+      "name": "Caster",
+      "type": "Server.Mobile",
+      "rule": "SerializableInterfaceMigrationRule"
+    },
+    {
+      "name": "End",
+      "type": "System.DateTime",
+      "rule": "PrimitiveTypeMigrationRule",
+      "ruleArguments": [
+        "DeltaTime"
+      ]
+    }
+  ]
+}