feat(Guild): add mfaLevel property (#2451)

src/structures/Guild.js

@@ -158,6 +158,12 @@ class Guild {
      */
     this.explicitContentFilter = data.explicit_content_filter;
 
+    /**
+     * The required MFA level for the guild
+     * @type {number}
+     */
+    this.mfaLevel = data.mfa_level;
+
     /**
      * The timestamp the client user joined the guild at
      * @type {number}