feat(xep): Add an event for when someone leaves the MUC

packages/moxxmpp/lib/src/xeps/xep_0045/events.dart

@@ -24,6 +24,7 @@ class MemberJoinedEvent extends XmppEvent {
   final RoomMember member;
 }
 
+/// Triggered when an entity changes their presence in the MUC.
 class MemberChangedEvent extends XmppEvent {
   MemberChangedEvent(this.roomJid, this.member);
 
@@ -33,3 +34,14 @@ class MemberChangedEvent extends XmppEvent {
   /// The new member.
   final RoomMember member;
 }
+
+/// Triggered when an entity leaves the MUC.
+class MemberLeftEvent extends XmppEvent {
+  MemberLeftEvent(this.roomJid, this.nick);
+
+  /// The JID of the room.
+  final JID roomJid;
+
+  /// The nick of the user who left.
+  final String nick;
+}

packages/moxxmpp/lib/src/xeps/xep_0045/xep_0045.dart

@@ -326,6 +326,12 @@ class MUCManager extends XmppManagerBase {
           'Should not receive unavailable with role="none" while joining',
         );
         room.members.remove(from.resource);
+        getAttributes().sendEvent(
+          MemberLeftEvent(
+            bareFrom,
+            from.resource,
+          ),
+        );
       } else {
         final member = RoomMember(
           from.resource,