Update publish step to handle pre-release

Author: andyleejordan

.vsts-ci/templates/publish-markets.yml

@@ -7,7 +7,15 @@ steps:
 
 - pwsh: |
     npm ci --loglevel=error
-    npm run publish -- --packagePath $(Pipeline.Workspace)/vscode-powershell/powershell-*.vsix --pat $(VsceToken)
+    $PackageJson = Get-Content -Raw $(Build.SourcesDirectory)/package.json | ConvertFrom-Json
+    $publishArgs = @(
+      if ($PackageJson.preview) { "--pre-release" }
+      "--packagePath"
+      "$(Pipeline.Workspace)/vscode-powershell/powershell-*.vsix"
+      "--pat"
+      "$(VsceToken)"
+    )
+    npm run publish -- @publishArgs
   displayName: Publishing VSIX to VS Code Marketplace
 
 # NOTE: We rarely update this script, so we can ignore errors from the gallery