yjs v13.6.13 breaks y-utility/y-multidoc-undomanager

[acnebs]

Describe the bug
Changes made in yjs v13.6.13 break the YMultiDocUndoManager found in y-utility.

I haven't had time to look through the code of each to identify the cause more directly, but wanted to go ahead and make sure this issue was known.

To Reproduce
Steps to reproduce the behavior:

1. Install yjs > 13.6.12
2. Install and use latest YMultiDocUndoManager
3. Trigger an undo from YMultiDocUndoManager
4. The entire undo stack is undone and wiped, not the last stack item

Expected behavior
Triggering YMultiDocUndoManager.undo should undo a single stack item, not the entire stack.

Environment Information

• yjs v13.6.13
• y-utility v0.1.4