src/vs/workbench/contrib/notebook/browser/viewModel/foldingModel.ts/FoldingModel/getMemento
class FoldingModel:	getMemento(): ICellRange[] {
		const collapsedRanges: ICellRange[] = [];
		let i = 0;
		while (i < this._regions.length) {
			const isCollapsed = this._regions.isCollapsed(i);

			if (isCollapsed) {
				const region = this._regions.toRegion(i);
				collapsedRanges.push({ start: region.startLineNumber - 1, end: region.endLineNumber - 1 });
			}

			i++;
		}

		return collapsedRanges;
	}