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.Core/Entities/Channels/IChannel.cs

@@ -8,6 +8,9 @@ namespace Discord
         /// <summary> Gets the name of this channel. </summary>
         string Name { get; }
 
+        /// <summary> Checks if the channel is NSFW. </summary>
+        bool IsNsfw { get; }
+
         /// <summary> Gets a collection of all users in this channel. </summary>
         IAsyncEnumerable<IReadOnlyCollection<IUser>> GetUsersAsync(CacheMode mode = CacheMode.AllowDownload, RequestOptions options = null);