fix: Crosspost throwing InvalidOperationException (#1671)

* Add INewsChannel

* Renaming variable to match the new type

src/Discord.Net.Core/Entities/Channels/INewsChannel.cs

@@ -0,0 +1,9 @@
+namespace Discord
+{
+    /// <summary>
+    ///     Represents a generic news channel in a guild that can send and receive messages.
+    /// </summary>
+    public interface INewsChannel : ITextChannel
+    {
+    }
+}