[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/Channels/ChannelHelper.cs

@@ -91,6 +91,8 @@ namespace Discord.Rest
                         Deny = overwrite.Permissions.DenyValue.ToString()
                     }).ToArray()
                     : Optional.Create<API.Overwrite[]>(),
+                SlowModeInterval = args.SlowModeInterval,
+                IsNsfw = args.IsNsfw,
             };
             return await client.ApiClient.ModifyGuildChannelAsync(channel.Id, apiArgs, options).ConfigureAwait(false);
         }