Run the UI tests with the COOP/COEP headers

[jtpio]

Fixes #93

This also helps prepare for #102, where two different libraries will be used for the communication between the main thread and the worker.

Changes

• Add a new webServer to the Playwright config
• Test accessing the site on http://localhost:8080 (COOP / COEP headers enabled)
• Test accessing files from the kernel in both modes

[jtpio]

CONSOLE MSG --- File system calls over Atomics.wait is only supported with jupyterlite>=0.4.0a3

The UI test with the stable jupyterlite-core is failing, which is expected.

[jtpio]

This should be good for now, and CI is now passing.

[martinRenou]

Thanks!