[dev]: enable end-to-end test suite on Safari and Firefox

[jsjoeio]

Currently, we only run end-to-end tests on Chrome. However, playwright supports all modern browsers:

Single API to automate Chromium, Firefox and WebKit.

After shipping a feature that causes code-server to not work in Safari, we think it's time to enable this.

Related:

• New 3.9.2 not work on iPadOS, Safari #2975
• https://github.com/microsoft/playwright/discussions/5991
• waiting on next release of playwright which should include a fix for: [BUG] WebKit on Mac crashes when RTCPeerConnection is used. microsoft/playwright#6009