refactor: improve regression testing

[SethFalco]

• Update Playwright, pngjs, and pixelmatch.
  • Unfortunately, this slows down CI a little, but it resolves 7 of the broken images. 🎉
• Rather than tracking that we skipped certain files, let's not extract them in the first place.
• Make better use of async/await.
  • This change alone speeds up local W3C test runs by 13% for me when tested with the same version of the dependencies.
• Only set the viewport once per page once rather than once per image.
• Replace some code with modern Node.js APIs.
  • Import fs/promises rather than do fs.promise.xyz everywhere.
  • Replace how we find files with Nodes standard library. (Requires Node.js v18+.)
  • Note: The Node.js v18 API is relied on in tests only. SVGO still supports v14 for now.
• A few other minor changes.