Add E2E tests using Cypress.io

[avanslaars]

Cypress.io is an e2e testing tool that will be open sourced soon. I think the Elm community could benefit from such a tool and thought this would be a good place to make that introduction.

I took the liberty of writing some tests for this, just to show how straight-forward writing e2e tests is with Cypress.

Aside from adding some files, these tests don't require any other changes to this project. All the existing code and structure remains the same and the tests can be run with the standalone Cypress GUI. This avoids the need for any node modules or for a server component to be built-in to the project.

Cypress tests are readable and the docs are solid. The tests run very quickly and can be run during development, so once the Elm compiler is happy with the code, you can quickly verify that the full user experience is working as intended.

[bahmutov]

As a representative of the Cypress.io team, I would love to see this merged in. We stand behind our OSS test runner and believe Elm community would benefit greatly from catching errors that can only be found when testing the entire application: misconfiguration, deployment errors, missing libraries, etc.

Especially since @rtfeldman got a Cypress 👕... there are no blockers 😉