Sometimes, instead of creating a new cell from scratch, it can be more convenient to duplicate an existing cell and then modify it. That will likely become more true over time as the data specified within a single cell becomes more complicated. Notion has this feature throughout their UI.

This issue is more subtle than #150 because it is incorrect to a duplicate a cell merely by deep copying it. In addition, the cell needs a new cell ID (a UUID) and in the case of formal cells, the formal content typically has its own UUID that must regenerated as well. One possible strategy is to extend the CellConstructor type with a new field providing a duplicate function, so that each cell type knows how to safely duplicate itself.