webapi: Add context when parsing HTML

Parsing HTML has different modes. Parsing HTML when setting innerHTML can be
different than parsing HTML inside a template, can be different than ...

html5ever handles this for us, provided we give it the correct context (the
tag name that the content is being parsed into). This adds an optional *Element
to the Parser which is fed into html5ever (its tagname).

Fixes: https://github.com/lightpanda-io/browser/issues/3336