Add a test converting HTML to attributed strings in StyledTextOverSK

The HTML rendering is painfully slow.

Test using [init(html:baseURL:documentAttributes:)](https://developer.apple.com/documentation/foundation/nsattributedstring/1524624-init) to create an attributed string from raw HTML. That would allow the attributed string to be serialised so the slowness is at creation time in HTML rather than on display. (Not relevant to this sample but highly relevant to the main use of these techniques).

Note the warnings in older versions in [this SO thread](https://stackoverflow.com/questions/4217820/convert-html-to-nsattributedstring-in-ios)