Replace Appveyor windows testing with GHA (#5599)

This PR moves us off of Appveyor for windows testing.

We are now doing windows/linux testing on GHA. With the exception of iojs, which we are only testing on Linux and have split out to it's own workflow.

We have also added npm-shrinkwrap.json to our gitignore, in order to not have to configure npm in CI to ignore it. If it's never checked in, it shouldn't exist in CI as you need to go out of your way to create it w/ npm.