feature: Add GetCategory method (#1261)

src/Discord.Net.WebSocket/Entities/Guilds/SocketGuild.cs

@@ -520,6 +520,15 @@ namespace Discord.WebSocket
         /// </returns>
         public SocketVoiceChannel GetVoiceChannel(ulong id)
             => GetChannel(id) as SocketVoiceChannel;
+        /// <summary>
+        ///     Gets a category channel in this guild.
+        /// </summary>
+        /// <param name="id">The snowflake identifier for the category channel.</param>
+        /// <returns>
+        ///     A category channel associated with the specified <paramref name="id" />; <c>null</c> if none is found.
+        /// </returns>
+        public SocketCategoryChannel GetCategoryChannel(ulong id)
+            => GetChannel(id) as SocketCategoryChannel;
 
         /// <summary>
         ///     Creates a new text channel in this guild.