Add support for Cypress version 16

[wlsf82]

Cypress launched version 16 (see the Changelog), and cypress-axe still supports only versions 10-15.

Please add support for version 16.

[esetnik]

Opened #193 for this.

It widens the cypress peer range to include ^16 and adds 16 to the CI matrix so the support is exercised rather than only declared, mirroring #187 for Cypress 15.

Verified locally against cypress@16.0.0 on Chrome 152 / Node 24 — the existing suite passes unchanged, and none of the APIs removed in v16 (Cypress.env(), cy.exec(), cy.end()) are used by this package.

🤖 Generated with Claude Code

[wlsf82]

@esetnik, when will your merged PR be released in a new version of cypress-axe?

Cc. @bmordue