Add Playwright to Codecov

[mnholtz]

We'd like a way to systematically calculate code coverage from e2e tests to facilitate discussion around minimum test expectations.

Playwright doesn't seem to provide code coverage support out of the box: https://github.com/microsoft/playwright/issues/9208

Potentially useful resources:

• https://github.com/microsoft/playwright/issues/13452
• https://playwright.dev/docs/api/class-coverage
• https://github.com/mxschmitt/playwright-test-coverage

[twschiller]

This will be nice, but given that we're working through the list of manual regression tests, the coverage of test cases/stores will be a better metric of coverage than code coverage. So I'd prioritize any blockers to testing over collecting code coverage