Add IChannel.Nsfw, RequireNsfw precondition (#633)

* Add IChannel.Nsfw, RequireNsfw precondition

* Refactored IChannel.Nsfw to IsNsfw, removed NsfwUtils

Per pull-request feedback

* proper nsfw channel check

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

@@ -289,5 +289,10 @@ namespace Discord.Rest
                 author = RestUser.Create(client, guild, model, webhookId);
             return author;
         }
+
+        public static bool IsNsfw(IChannel channel) =>
+            IsNsfw(channel.Name);
+        public static bool IsNsfw(string channelName) =>
+            channelName == "nsfw" || channelName.StartsWith("nsfw-");
     }
 }

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

@@ -46,6 +46,7 @@ namespace Discord.Rest
 
         //IChannel
         string IChannel.Name => null;
+        bool IChannel.IsNsfw => ChannelHelper.IsNsfw(this);
 
         Task<IUser> IChannel.GetUserAsync(ulong id, CacheMode mode, RequestOptions options)
             => Task.FromResult<IUser>(null); //Overriden

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

@@ -97,6 +97,7 @@ namespace Discord.Rest
 
         //IChannel
         string IChannel.Name { get { throw new NotSupportedException(); } }
+        bool IChannel.IsNsfw { get { throw new NotSupportedException(); } }
         IAsyncEnumerable<IReadOnlyCollection<IUser>> IChannel.GetUsersAsync(CacheMode mode, RequestOptions options)
         {
             throw new NotSupportedException();