Variable yUndoPluginKey`Const`

yUndoPluginKey: PluginKey<
    {
        hasRedoOps: boolean;
        hasUndoOps: boolean;
        prevSel: RelativeSelection
        | null;
        undoManager: UndoManager;
    },
> = ...

Our own plugin key for the per-doc undo plugin. Kept separate from @tiptap/y-tiptap's yUndoPluginKey so our plugins are never confused with upstream ones.