Upgrade Cypress to version 15

[bact]

• Upgrade Cypress to 15.3.0
• Add Cypress config file: cypress.config.js (required by Cypress 10+)
  • Make the config using the existing directory structure
• Use setup-node action built-in cache (removed from the PR, only works if package-lock.json exists)
• Update CI (and README) to rely on Cypress version as specified in the package.json (fewer places to update the version string)

[bact]

re: package-lock.json. npm ci requires it. Will change to npm install instead.

[progval]

Thanks!