These changes apply CSS classes SVG highlight rects, which will allow
for clustered-highlight styling.

With these changes:

* `hypothesis-highlight` elements always have a `hypothesis-highlight`
  CSS class, similarly to how SVG highlight rects have always had a
  `hypothesis-svg-highlight` class.
* CSS classes passed to `highlightRange` extend instead of replace CSS
  classes on `hypothesis-highlight` elements.
* CSS classes passed to `highlightRange` are also added to
  `hypothesis-svg-element` rects.