infra: add ci and testing

My config seems to fail mysteriously at some point during testing. Later on I found it to be the culprit of mishandling archives between `.VSIXPackage` and `.vsix`, but this only happens on CI.

Maybe because I only used parts of the extensions that are available at https://github.com/danielphan2003/nixpkgs, and I barely do local testing so I didn't encounter it.