Expand/Collapse should be announced

[terrimorton]

This issue was raised by the C&I team doing an accessibility review of the .NET website. Reference GitHub issue 1043

Environment Details:
Application Name : DotNETWebsite
Application URL: https://dotnet.microsoft.com/
Operating System: Windows 10
Browser: Edge 44.17763.1.0
Usability Issue (Narrator)

Repro Steps:

1. Launch the application using Url: https://dotnet.microsoft.com/
2. Navigate and click on .NET link-->Languages-->Learn about >NET languages (https://dotnet.microsoft.com/languages)
3. Start Narrator.
4. Click on code folding button which is present inside editor section
5. Verify Narrator not announce any information of Collapsed/Expand for Code folding button.

Actual:
When we click on code folding button which is present inside editor section, button gets Expanded/Collapsed but narrator not announces any information of Collapsed/Expand for Code folding button.

Expected:
Narrator should announce information of Collapsed/Expand for Code folding button.

User Impact:
Screen reader users finds difficult to identify the clear feedback on the prior action.

Attachment for Reference:
A11y_DotNETWebsite_DotNetProgrammingLanguages_Usability.pptx

Examples of pages with bug:
https://dotnet.microsoft.com/languages
https://dotnet.microsoft.com/learn/dotnet/in-browser-tutorial/1

[alexdima]

Let's track in microsoft/vscode#43262