Insert global stylesheets into document.adoptedStyleSheets

[nolanlawson]

We currently cannot insert styles to document.adoptedStyleSheets, because for backwards compatibility we need to continue appending global stylesheets as <style>s to the document.head.

However, there is some data showing that document.adoptedStyleSheets is a performance win (#2683 #2460), and it's cleaner in our code to just use one system both at the global level and at the local (shadow root) level, so it would be beneficial to use document.adoptedStyleSheets.

[nolanlawson]

Well #2923 actually showed there's a slight perf improvement for using <style>s in Chrome anyway, so this may need to be re-evaluated in the future. It makes no sense to me that the ceremony of creating a DOM node just to add a style actually improves perf relative to constructable stylesheets, but maybe Chrome has optimized one case more than the other.

[git2gus]

This issue has been linked to a new work item: W-16535766