Add Cypress e2e to test and automatically generate the README's example GIF

[emibcn]

As this is going to a fast paced project (sic), manually generating the GIF and the end-to-end testing will eventually be an stopper.

Proposal

• Use Cypress with free tier: only 1 worker in the GitHub Workflow, no Cypress SaaS needed.
• Add a dedicated directory to store all Cypress test files.
• Do unit tests (some hard to test components from non e2e tests) and/or component compositions. Save as disposable artifacts.
• Do example/tutorial/README integration test. Save as GIF into the badges branch.
• Allow developers to execute e2e in development environment (as a package.json script?).

Note: This issue can be handled by more than one person and split into sub-issues:

• Create Cypress environment: system and NPM dependencies (docker&docker-compose?), generate cypress config/ini files
• Create the unit/composite tests
• Create the tutorial test
• Convert the tutorial test to GIF
• Add tests and GIF creation to GitHub Workflow
• Ease development environment execution

If you want to do just a sub-issue, create it by yourself and link it here.