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.Core/Entities/Guilds/GuildProperties.cs

@@ -1,3 +1,5 @@
+using System.Globalization;
+
 namespace Discord
 {
     /// <summary>
@@ -84,5 +86,23 @@ namespace Discord
         ///     are enabled, without the need to manipulate the logic of the flag.
         /// </remarks>
         public Optional<SystemChannelMessageDeny> SystemChannelFlags { get; set; }
+        /// <summary>
+        ///     Gets or sets the preferred locale of the guild in IETF BCP 47 language tag format.
+        /// </summary>
+        /// <remarks>
+        ///     This property takes precedence over <see cref="PreferredCulture"/>.
+        ///     When it is set, the value of <see cref="PreferredCulture"/>
+        ///     will not be used.
+        /// </remarks>
+        public Optional<string> PreferredLocale { get; set; }
+        /// <summary>
+        ///     Gets or sets the preferred locale of the guild.
+        /// </summary>
+        /// <remarks>
+        ///     The <see cref="PreferredLocale"/> property takes precedence
+        ///     over this property. When <see cref="PreferredLocale"/> is set,
+        ///     the value of <see cref="PreferredCulture"/> will be unused.
+        /// </remarks>
+        public Optional<CultureInfo> PreferredCulture { get; set; }
     }
 }

src/Discord.Net.Core/Entities/Guilds/IGuild.cs

@@ -1,6 +1,7 @@
 using Discord.Audio;
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.Threading.Tasks;
 
 namespace Discord
@@ -249,6 +250,24 @@ namespace Discord
         /// </returns>
         int PremiumSubscriptionCount { get; }
 
+        /// <summary>
+        ///     Gets the preferred locale of this guild in IETF BCP 47
+        ///     language tag format.
+        /// </summary>
+        /// <returns>
+        ///     The preferred locale of the guild in IETF BCP 47
+        ///     language tag format.
+        /// </returns>
+        string PreferredLocale { get; }
+
+        /// <summary>
+        ///     Gets the preferred culture of this guild.
+        /// </summary>
+        /// <returns>
+        ///     The preferred culture information of this guild.
+        /// </returns>
+        CultureInfo PreferredCulture { get; }
+
         /// <summary>
         ///     Modifies this guild.
         /// </summary>