Add automatic publishing of vsix to marketplace

[gep13]

Currently, the vsix is only published to MyGet. On a tagged build, we should also publish the vsix file to the Marketplace. This can be done using the VsixPublisher.exe:

https://www.meziantou.net/ci-cd-pipeline-for-a-visual-studio-extension-vsix-using-azure-devops.htm

[nils-a]

There is also the Open VSIX Gallery that many projects use to release preview versions.