When highlighting a run of adjacent text nodes, wrap the group of
adjacent text nodes in a single `<hypothesis-highlight>` element rather
than wrapping each text node separately. Adjacent text nodes may be
created if:

 - A sub-range of the text is highlighted by Hypothesis, and the
   highlight is later removed, for example when switching groups.
 - The text nodes were generated programatically by JavaScript rather
   than by the browser's HTML parser

For screen reader users, this reduces the number of "annotation
start"/"annotation end" comments they will hear in this scenario. It is
also likely to benefit other code which interacts with highlighted text.