Adopt @playwright/test and use JSHandle

[Tyriar]

I'm using the new @playwright/test package in Luna Paint and it's essentially just doing what we're doing but in a much nicer way so we should adopt it. Additionally we should move away from just evaluating strings in page.evaluate to fetching a JSHandle to the xterm API and using that to test which would give us strong typing on the renderer side.

[Tyriar]

• Sharing page contexts across tests https://playwright.dev/docs/test-parallel/#reusing-single-page-between-tests
• Having playwright manage the server https://playwright.dev/docs/test-advanced/#launching-a-development-web-server-during-the-tests

[Tyriar]

This is done for the most part

[mxschmitt]

Great work @Tyriar!