Not implementing onDidSelectItem for now. (#45589)

src/vs/vscode.d.ts

@@ -1569,6 +1569,7 @@ declare module 'vscode' {
 
 		/**
 		 * An optional function that is invoked whenever an item is selected.
+		 * (Only honored when the picker does not allow multiple selections.)
 		 */
 		onDidSelectItem?(item: QuickPickItem | string): any;
 	}