Add Office 2016 support, bump to v1.20

Office 2016 Standard, Professional Plus, and Home & Business editions
added with PerpetualVL2019 channel for volume licenses.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Models/OfficeVersion.cs

@@ -4,6 +4,7 @@ namespace InstaSoftOfficeTool.Models
     {
         Office2024,
         Office2021,
-        Office2019
+        Office2019,
+        Office2016
     }
 }