Custom Thunderbird Add-On with Web Extensions

I need a Thunderbird 60.4.0 add-on that implements Web Extensions instead of the legacy XUL/Overlay. It must be implemented as a toolbar button in the Compose Message window that performs three basic tasks: 1) runs an external Python script with a Native Messaging connection to retrieve information 2) post a segment of that information in the Compose Message status bar, and 3) print data in the message body. Firefox is moving away from XUL/overlay and going exclusively to Web Extensions, but documentation is sparse for Thunderbird. Web Extensions are the only valid way, moving forward, to implement Native Messaging with external apps.