cellCount and cellAt API proposal so that notebook aligns better with text document

src/vs/vscode.proposed.d.ts

@@ -1066,11 +1066,23 @@ declare module 'vscode' {
 		// todo@API should we really expose this?
 		readonly viewType: string;
 
-		// todo@API add cellAt(index): NotebookCell
-		// todo@API add cellCount: number;
+		// todo@API cellsAt(range)? getCell(index>)?
 		/** @deprecated Use `getCells(<...>) instead */
 		readonly cells: ReadonlyArray<NotebookCell>;
 
+		/**
+		 * The number of cells in the notebook document.
+		 */
+		readonly cellCount: number;
+
+		/**
+		 * Return the cell at the specified index. The index will be adjusted to the notebook.
+		 *
+		 * @param index - The index of the cell to retrieve.
+		 * @return A [cell](#NotebookCell).
+		 */
+		cellAt(index: number): NotebookCell;
+
 		/**
 		 * Get the cells of this notebook. A subset can be retrieved by providing
 		 * a range. The range will be adjuset to the notebook.

src/vs/workbench/api/common/extHostNotebookDocument.ts

@@ -179,8 +179,12 @@ export class ExtHostNotebookDocument extends Disposable {
 				get isUntitled() { return that.uri.scheme === Schemas.untitled; },
 				get isClosed() { return that._disposed; },
 				get metadata() { return that._metadata; },
-				set metadata(_value: Required<vscode.NotebookDocumentMetadata>) { throw new Error('Use WorkspaceEdit to update metadata.'); },
 				get cells(): ReadonlyArray<vscode.NotebookCell> { return that._cells.map(cell => cell.cell); },
+				get cellCount() { return that._cells.length; },
+				cellAt(index) {
+					index = that._validateIndex(index);
+					return that._cells[index].cell;
+				},
 				getCells(range) {
 					const cells = range ? that._getCells(range) : that._cells;
 					return cells.map(cell => cell.cell);
@@ -232,6 +236,16 @@ export class ExtHostNotebookDocument extends Disposable {
 		}
 	}
 
+	private _validateIndex(index: number): number {
+		if (index < 0) {
+			return 0;
+		} else if (index >= this._cells.length) {
+			return this._cells.length - 1;
+		} else {
+			return index;
+		}
+	}
+
 	private _validateRange(range: vscode.NotebookCellRange): vscode.NotebookCellRange {
 		if (range.start < 0) {
 			range = range.with({ start: 0 });