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* remove (very outdated) todo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.Rest/Entities/Channels/RestTextChannel.cs

@@ -17,7 +17,7 @@ namespace Discord.Rest
         /// <inheritdoc />
         public string Topic { get; private set; }
         /// <inheritdoc />
-        public int SlowModeInterval { get; private set; } 
+        public virtual int SlowModeInterval { get; private set; } 
         /// <inheritdoc />
         public ulong? CategoryId { get; private set; }