add build vsix

Author: bschnurr

build/azure-devdiv-pipeline.pre-release.yml

@@ -97,6 +97,9 @@ extends:
               - script: npm run package
                 displayName: Build extension
 
+              - script: npm run vsce-package
+                displayName: Build VSIX
+
               - powershell: |
                   cp "$(Build.StagingDirectory)\autopep8.vsix" "$(Build.StagingDirectory)\drop\autopep8.vsix"
                 displayName: Copy VSIX into drop