feat(JitsiConference): add getLastN

JitsiConference.js

@@ -972,6 +972,14 @@ JitsiConference.prototype.pinParticipant = function(participantId) {
     this.rtc.pinEndpoint(participantId);
 };
 
+/**
+ * Obtains the current value for "lastN". See {@link setLastN} for more info.
+ * @returns {number}
+ */
+JitsiConference.prototype.getLastN = function() {
+    return this.rtc.getLastN();
+};
+
 /**
  * Selects a new value for "lastN". The requested amount of videos are going
  * to be delivered after the value is in effect. Set to -1 for unlimited or