[Feature] Enable Istanbul code coverage collection with Playwright Test

[anishkny]

For frontend source code instrumented with babel-plugin-istanbul for code coverage, allow an easy way to collect that code coverage via nyc. The plugin stores coverage data in the window.__coverage__ variable in the browser.

For now, this can be done in user-land via the playwright-test-coverage package.

Ideally this should be built into Playwright test runner.

See also: mxschmitt/playwright-test-coverage#1

[mxschmitt]

Will reopen #7030 instead of having a new issue.