Fix #995 ICategoryChannel.CategoryID throws NotSupportedException

src/Discord.Net.Rest/Entities/Channels/RestCategoryChannel.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
@@ -25,6 +25,8 @@ namespace Discord.Rest
         private string DebuggerDisplay => $"{Name} ({Id}, Category)";
 
         // IGuildChannel
+        ulong? IGuildChannel.CategoryId
+            => throw new NotSupportedException();
         IAsyncEnumerable<IReadOnlyCollection<IGuildUser>> IGuildChannel.GetUsersAsync(CacheMode mode, RequestOptions options)
             => throw new NotSupportedException();
         Task<IGuildUser> IGuildChannel.GetUserAsync(ulong id, CacheMode mode, RequestOptions options)

src/Discord.Net.WebSocket/Entities/Channels/SocketCategoryChannel.cs

@@ -51,6 +51,8 @@ namespace Discord.WebSocket
         internal new SocketCategoryChannel Clone() => MemberwiseClone() as SocketCategoryChannel;
 
         // IGuildChannel
+        ulong? IGuildChannel.CategoryId
+            => throw new NotSupportedException();
         IAsyncEnumerable<IReadOnlyCollection<IGuildUser>> IGuildChannel.GetUsersAsync(CacheMode mode, RequestOptions options)
             => ImmutableArray.Create<IReadOnlyCollection<IGuildUser>>(Users).ToAsyncEnumerable();
         Task<IGuildUser> IGuildChannel.GetUserAsync(ulong id, CacheMode mode, RequestOptions options)