Migrate Windows build to GitHub Actions

[ForNeVeR]

TODO:

• build
• test
• release (the part that's already implemented for Linux and macOS)
• remove Appveyor (don't forget to clean up the outdated scripts)
  • will be resolved in the wider scope of Port to GitHub Actions #24
• test that everything's green
• test the release workflow separately (don't forget to disable Appveyor temporarily beforehand)
• drop temporary commit(s)
• verify that the bundled file set is the same
• check the dependencies of the resulting artifact (should be the same as before)
• review everything