Error encountered when inserting multiple columns into multiple columns

[freesuton]

Describe the bug When inserting a multi-column in a multi-column, it shows

To Reproduce [<clear steps to reproduce are super helpful! Best is to provide an online sandbox, click to create one> ](https://www.blocknotejs.org/examples/basic/multi-column)

Try in the multi-column page.

[matthewlipski]

I think we probably want to change this to only allow columnLists at the top level, so not nested within other blocks or columns (and have the slash menu reflect that). Right now it's also still possible to create them as children of blocks.