also adds two plural forms that were missing

It has new API which we want to use.

This got added in scoped-element 2.1.0 and is more or less the same
as getShadowRootDocument()

This is inspired by how lion web components handle this, and much cleaner.

Newest Ubuntu switched Firefox to a snap, which breaks karma.

Try to use playwright to download and cache firefox/chromium
and use those for karma.

Compared to puppeteer where we did this some time ago this caches
the browsers globally, and only downloads them when we start the tests.