src/vs/workbench/contrib/notebook/browser/notebookEditorWidget.ts/NotebookEditorWidget/splitNotebookCell
class NotebookEditorWidget:	async splitNotebookCell(cell: ICellViewModel): Promise<CellViewModel[] | null> {
		if (!this.viewModel) {
			return null;
		}

		if (this.viewModel.options.isReadOnly) {
			return null;
		}

		const index = this.viewModel.getCellIndex(cell);

		return this.viewModel.splitNotebookCell(index);
	}