Add Cypress test framework

[Decipher]

Is your feature request related to a problem? Please describe. Unit tests aren't enough to ensure that Druxt is working. End 2 end tests need to be added.

Describe the solution you'd like

• [x] Add Cypress tests to test Nuxt integrations.
  • [x] Add ~test:cypress~ test:e2e script that: ~- [ ] Ensures a test backend is running and installed (ddev start / ddev drupal-install)~
    • [x] Starts various frontends and runs various end-to-end tests
  • [x] #562
  • [ ] Create test suite for each example:
    • [ ] Custom Module
    • [ ] DruxtSite
      • [x] #566
    • [ ] EntityForm
• [ ] #560

Describe alternatives you've considered N/A

Additional context https://docs.cypress.io/guides/component-testing/framework-configuration#Nuxt

[Decipher]

I've demoted this issue from the 1.0.0 milestone as there's a significant amount of setup required to get it working, both locally and on CI.

It would be a nice to have.