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* remove (very outdated) todo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/RestNewsChannel.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Model = Discord.API.Channel;
+
+namespace Discord.Rest
+{
+    /// <summary>
+    ///     Represents a REST-based news channel in a guild that has the same properties as a <see cref="RestTextChannel"/>.
+    /// </summary>
+    [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
+    public class RestNewsChannel : RestTextChannel
+    {
+        internal RestNewsChannel(BaseDiscordClient discord, IGuild guild, ulong id)
+            :base(discord, guild, id)
+        {
+        }
+        internal new static RestNewsChannel Create(BaseDiscordClient discord, IGuild guild, Model model)
+        {
+            var entity = new RestNewsChannel(discord, guild, model.Id);
+            entity.Update(model);
+            return entity;
+        }
+        public override int SlowModeInterval => throw new NotSupportedException("News channels do not support Slow Mode.");
+        public override Task AddPermissionOverwriteAsync(IRole role, OverwritePermissions permissions, RequestOptions options = null)
+        {
+            throw new NotSupportedException("News channels do not support Overwrite Permissions.");
+        }
+        public override Task AddPermissionOverwriteAsync(IUser user, OverwritePermissions permissions, RequestOptions options = null)
+        {
+            throw new NotSupportedException("News channels do not support Overwrite Permissions.");
+        }
+        public override OverwritePermissions? GetPermissionOverwrite(IRole role)
+        {
+            throw new NotSupportedException("News channels do not support Overwrite Permissions.");
+        }
+        public override OverwritePermissions? GetPermissionOverwrite(IUser user)
+        {
+            throw new NotSupportedException("News channels do not support Overwrite Permissions.");
+        }
+        public override Task RemovePermissionOverwriteAsync(IRole role, RequestOptions options = null)
+        {
+            throw new NotSupportedException("News channels do not support Overwrite Permissions.");
+        }
+        public override Task RemovePermissionOverwriteAsync(IUser user, RequestOptions options = null)
+        {
+            throw new NotSupportedException("News channels do not support Overwrite Permissions.");
+        }
+    }
+}