Messenger Javascript API open new message method

Add new method to open the latest conversation with a new message.

This is useful when there is an unread message and you want to open the message directly in the widget.

For example you could have a custom button with an unread notification. When clicked it should either open the main chat (if there are multiple unread chats) or open a specific unread chat.

An alternative is to improve the unread callback data to include information about which chat ids have unread messages.

https://help.featurebase.app/en/help/articles/9916388-messenger-javascript-api-methods