Move Windows CI to GitHub Actions

[jameslamb]

Today, doppel-cli is tested on Windows using AppVeyor. AppVeyor is annoying for a few reasons. Most importantly, AppVeyor only allows a single job at a time.

To close this issue, propose a GitHub Actions template that does what .appveyor.yml does. You can use this for inspiration:

• https://github.com/microsoft/LightGBM/blob/master/.github/workflows/main.yml
• https://github.com/microsoft/LightGBM/blob/master/.ci/test_r_package_windows.ps1