Hi Brad,
A bit of a late response, but I found this live demo that shows how to work with TS when developing widgets -
YouTube
You can use the Yeoman generator tool to quickly start with new widgets in modern es6 and wrap it with TS -
GitHub - Esri/generator-esri-appbuilder-js: Yeoman generator to help customize Esri's WebAppBuilder
Shay
Shay