[Feature] Text-In-Stage support & missing IVoiceChannel properties (#2610)

* initial commit

* add a check for voice/stage channels in `PinAsync`

* moar refactoring

* Update IStageChannel.cs

* forgot about mocked. again.

src/Discord.Net.Rest/Entities/Messages/MessageHelper.cs

@@ -212,6 +212,8 @@ namespace Discord.Rest
         public static async Task PinAsync(IMessage msg, BaseDiscordClient client,
             RequestOptions options)
         {
+            if (msg.Channel is IVoiceChannel)
+                throw new NotSupportedException("Pinned messages are not supported in text-in-voice channels.");
             await client.ApiClient.AddPinAsync(msg.Channel.Id, msg.Id, options).ConfigureAwait(false);
         }