Add GitHub action to build .vsix

[JamieMagee]

You can use the tfx-cli¹ to build and package the extension, as well as publish to the marketplace. There are currently some tasks for Azure DevOps which encapsulate many of these actions², but they can be replicated with tfx-cli and some small scripts.

Footnotes

1. https://www.npmjs.com/package/tfx-cli ↩

2. https://github.com/microsoft/azure-devops-extension-tasks ↩