Replace old markdown and remove redundant events (#389)

* Remove redundant messages

* Replaced old markdown with preview.
Added nuget config to add preview feed

* Fix old extensions usage

* Scrollbar margin for the markdown.

---------

Co-authored-by: Burak Kaan Köse <bkaankose@outlook.com>

Wino.Messages/Client/Shell/ShellStateUpdated.cs

@@ -1,7 +0,0 @@
-namespace Wino.Messaging.Client.Shell
-{
-    /// <summary>
-    /// When reading mail state or reader pane narrowed state is changed.
-    /// </summary>
-    public record ShellStateUpdated;
-}