Use a storage singleton

This way the instance can be reused, which allows access to non-deprecated `DataFormat` and `AssetType`.

src/lib/storage.js

@@ -26,4 +26,6 @@ class Storage extends ScratchStorage {
     }
 }
 
-export default Storage;
+const storage = new Storage();
+
+export default storage;

src/reducers/vm.js

 import VM from 'scratch-vm';
-import Storage from '../lib/storage';
+import storage from '../lib/storage';
 
 const SET_VM = 'scratch-gui/vm/SET_VM';
 const defaultVM = new VM();
-defaultVM.attachStorage(new Storage());
+defaultVM.attachStorage(storage);
 const initialState = defaultVM;
 
 const reducer = function (state, action) {