Make a Go module, fix up unit tests, have them run on every Pull Request

[phillip-stephens]

This PR does a couple things to get ZFlags in a better state

• Creates go.mod and go.sum so go tooling can run go build and go test easier
• Enable unit tests and a compile check on all PRs
• Fix up failing unit tests, including adding one for the specific issue encountered in Default arguments don't work properly with multiply declared groups #3
• Remove dependency on https://pkg.go.dev/github.com/jessevdk/go-flags to clean up go.mod