feature: Fix #1280 Add NewsChannel Types (#1293)

* add News channel type

* remove (very outdated) todo

* add [Socket/Rest]NewsChannel types

* update TextChannel properties to include a Type optional parameter with validation

as of writing, this feature is still only available to verified guilds, which makes it impossible for testing.

* actually create the news channels when given the type

* throw NotSupportedException in News channel

throw a NotSupportedException whenever trying to use SlowModeInterval or anything related to overwrite permissions

* make RestNewsChannel throw NotSupportedException also

* remove the (untested) ability to change channel types

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

@@ -12,6 +12,8 @@ namespace Discord
         /// <summary> The channel is a group channel. </summary>
         Group = 3,
         /// <summary> The channel is a category channel. </summary>
-        Category = 4
+        Category = 4,
+        /// <summary> The channel is a news channel. </summary>
+        News = 5
     }
 }