src/vs/workbench/contrib/notebook/browser/viewModel/notebookViewModelImpl.ts/NotebookViewModel/getFoldingStartIndex
class NotebookViewModel:	getFoldingStartIndex(index: number): number {
		if (!this._foldingRanges) {
			return -1;
		}

		const range = this._foldingRanges.findRange(index + 1);
		const startIndex = this._foldingRanges.getStartLineNumber(range) - 1;
		return startIndex;
	}