feat(no-export): disallow exports in files containing tests

[unlikelyzero]

Adds a no-export rule, ported from jest/no-export.

Playwright collects spec files by running them, not by importing anything they export, so an export in a spec file has no effect on the test run. It usually means a helper that belongs in a shared module, or a leftover from a file that used to be something else. It's also a subtle footgun: importing a spec file to reuse an export registers its tests a second time.

The rule only fires in files that contain at least one test or test.describe call, so fixture and helper modules are unaffected. ESM exports (export const, export default, export *, export { … }, export type), TypeScript's export =, and CommonJS module.exports / module.exports.foo are all covered. A locally declared module variable is not treated as CommonJS.

Not enabled in the recommended config, matching eslint-plugin-jest.