Essentials: SemanticScreenReader

[davidbritch]

There's a new class in Essentials called SemanticScreenReader - https://github.com/dotnet/maui/blob/main/src/Essentials/src/SemanticScreenReader/SemanticScreenReader.shared.cs

It has an Announce method that speaks the string passed to it. It will need doc'ing as part of Essentials.

Original engineering issue: dotnet/maui#1719

cc: @adegeo