I have a partial answer - you can create a Watchlist template in JSON - see examples from other Solutions: https://github.com/Azure/Azure-Sentinel/tree/c6dce9c3aa4d4b4d02423ac4eb5a6b677a39e432/Solutions/SOC-Process-Framework/Watchlists
The Folder structure doesn't list Watchlist, but you can create it just like you do for Workbooks or Parsers (or any other resource) https://github.com/Azure/Azure-Sentinel/tree/master/Solutions#guide-to-building-microsoft-sentinel-solutions
The Solution Package Tool, shows entries for Watchlists, so you can use this as a guide: https://github.com/Azure/Azure-Sentinel/blob/master/Tools/Create-Azure-Sentinel-Solution/V2/README.md
I hope this helps, please "accept" the answer if it does?