feat(User): add locale property (#2571)

src/structures/User.js

@@ -52,6 +52,13 @@ class User extends Base {
      */
     if (typeof data.avatar !== 'undefined') this.avatar = data.avatar;
 
+    /**
+     * The locale of the user's client (ISO 639-1)
+     * @type {?string}
+     * @name User#locale
+     */
+    if (data.locale) this.locale = data.locale;
+
     /**
      * The ID of the last message sent by the user, if one was sent
      * @type {?Snowflake}