Add IsNsfw to TextChannelProperties

src/Discord.Net.Core/Entities/Channels/TextChannelProperties.cs

@@ -7,5 +7,9 @@
         /// What the topic of the channel should be set to.
         /// </summary>
         public Optional<string> Topic { get; set; }
+        /// <summary>
+        /// Should this channel be flagged as NSFW?
+        /// </summary>
+        public Optional<bool> IsNsfw { get; set; }
     }
 }