Upgrade to Cypress 15

Chains the earlier majors: a project several majors behind needs only this recipe, which also runs the code changes of the earlier majors in order, and upgrades the dependencies once, straight to 15.x, so no intermediate version has to resolve. Upgrade cypress to 15.x in package.json. Rewrites cy.exec(...).its('code') to exitCode, cy.stub(o, n, fn) to .callsFake(fn) and Cypress.SelectorPlayground to Cypress.ElementSelector, and marks getSelector() and the other changes that need a decision (FindCypress15Changes). Not done by this recipe: Node.js 20 or later, dropping webpack 4 and Vite 4 support, the ESM-only @cypress/vite-dev-server, and .then(r => r.code) style access to the exec result.