Trigger `sendMessage` event

src/headless/converse-chat.js

@@ -997,6 +997,16 @@ converse.plugins.add('converse-chat', {
                     message = await this.createMessage(attrs);
                 }
                 api.send(this.createMessageStanza(message));
+
+               /**
+                * Triggered when a message is being sent out
+                * @event _converse#sendMessage
+                * @type { Object }
+                * @property { Object } data
+                * @property { (_converse.ChatBox | _converse.ChatRoom) } data.chatbox
+                * @property { (_converse.Message | _converse.ChatRoomMessage } data.message
+                */
+                api.trigger('sendMessage', {'chatbox': this, message});
                 return message;
             },