feat: add support for premium guilds (#3316)

* add premiumTier and premiumSubscriptionCount

* add premiumSinceTimestamp and premiumSince

* add premium message types

* typings

* add GuildEmoji#available

* fix doc description

src/structures/Guild.js

@@ -188,6 +188,30 @@ class Guild extends Base {
      */
     this.embedEnabled = data.embed_enabled;
 
+    /**
+     * The type of premium tier:
+     * * 0: NONE
+     * * 1: TIER_1
+     * * 2: TIER_2
+     * * 3: TIER_3
+     * @typedef {number} PremiumTier
+     */
+
+    /**
+     * The premium tier on this guild
+     * @type {PremiumTier}
+     */
+    this.premiumTier = data.premium_tier;
+
+    /**
+     * The total number of users currently boosting this server
+     * @type {?number}
+     * @name Guild#premiumSubscriptionCount
+     */
+    if (typeof data.premium_subscription_count !== 'undefined') {
+      this.premiumSubscriptionCount = data.premium_subscription_count;
+    }
+
     /**
      * Whether widget images are enabled on this guild
      * @type {?boolean}