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.Rest/API/Common/Guild.cs

@@ -58,5 +58,7 @@ namespace Discord.API
         public SystemChannelMessageDeny SystemChannelFlags { get; set; }
         [JsonProperty("premium_subscription_count")]
         public int? PremiumSubscriptionCount { get; set; }
+        [JsonProperty("preferred_locale")]
+        public string PreferredLocale { get; set; }
     }
 }

src/Discord.Net.Rest/API/Rest/ModifyGuildParams.cs

@@ -32,5 +32,7 @@ namespace Discord.API.Rest
         public Optional<ExplicitContentFilterLevel> ExplicitContentFilter { get; set; }
         [JsonProperty("system_channel_flags")]
         public Optional<SystemChannelMessageDeny> SystemChannelFlags { get; set; }
+        [JsonProperty("preferred_locale")]
+        public string PreferredLocale { get; set; }
     }
 }