Set Playwright and Puppeteer to use new headless mode by default

[dgrebb]

Let's make the full change in source to use new headless mode as the default.

Test regressions, test fallbacks, and ensure support for old mode is supported when new becomes vendor standard.

Notes

• Headless mode: deprecation warning after upgrading  garris/BackstopJS#1525 (comment)
• The Chromium team was supposed to default to new by EOY 2023
• there are reports that Playwright runs 40% slower with new headless

[dgrebb]

@garris let's discuss this - I see some evidence that new headless is actually slower.

It appears that one can pass headless: old as the mode, which apparently silences the deprecation warnings.

Maybe that's an option for now? The Chromium team says "we're going to make that the default later this year" back in January. So ¯_(ツ)_/¯

I'll look into ways perf testing can be done.

[garris]

Few thoughts.

1. pretty sure this is exposed in engineOptions so risk of defaulting this to the "wrong" setting is fairly low.
2. we should probably default this to headless: new but provide code snippet in docs for users who might hit an incompatibility.
3. backstop stashes time started and time completed and spits out duration into the CLI. So testing for performance should be pretty easy. You could run sanity test sequentially 20 times and grep for the timing value to get a cumulative average.
4. I'm open to other approaches. Just sharing gut reactions. Cheers.

[dgrebb]

Closed by garris#1535

Super happy I haven't caused any nightmares yet.