feature: #1381 Guild PreferredLocale support (#1387)

* Fix #1381 Guild PreferredLocale support

Adds support for getting and modifying a guild's preferred_locale. This is a language tag in IETF BCP 47 format, which works with the built-in CultureInfo.

While Discord only supports a number of cultures, I think that this restriction should be handled at the API and not by the wrapper. (Also easier on our end)

* Add PreferredCulture to IGuild

This property was defined in RestGuild and SocketGuild, so it only makes sense to make it part of IGuild as well.

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

@@ -5,6 +5,7 @@ using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Globalization;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -105,6 +106,11 @@ namespace Discord.WebSocket
         public string Description { get; private set; }
         /// <inheritdoc />
         public int PremiumSubscriptionCount { get; private set; }
+        /// <inheritdoc />
+        public string PreferredLocale { get; private set; }
+
+        /// <inheritdoc />
+        public CultureInfo PreferredCulture { get; private set; }
 
         /// <inheritdoc />
         public DateTimeOffset CreatedAt => SnowflakeUtils.FromSnowflake(Id);
@@ -374,6 +380,8 @@ namespace Discord.WebSocket
             SystemChannelFlags = model.SystemChannelFlags;
             Description = model.Description;
             PremiumSubscriptionCount = model.PremiumSubscriptionCount.GetValueOrDefault();
+            PreferredLocale = model.PreferredLocale;
+            PreferredCulture = new CultureInfo(PreferredLocale);
 
             if (model.Emojis != null)
             {