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.Rpc/Entities/Channels/RpcChannel.cs

@@ -1,4 +1,5 @@
-using System;
+using Discord.Rest;
+using System;
 
 using Model = Discord.API.Rpc.Channel;
 
@@ -7,6 +8,7 @@ namespace Discord.Rpc
     public class RpcChannel : RpcEntity<ulong>
     {
         public string Name { get; private set; }
+        public bool IsNsfw => ChannelHelper.IsNsfw(Name);
 
         public DateTimeOffset CreatedAt => SnowflakeUtils.FromSnowflake(Id);