Fix a bug that could cause pieces of text to be highlighted twice rather
than once. For this bug to occur:

 1. The anchored range had to start in the middle of a text node
 2. The anchored range had to end _after_ the text node, so that the
    range's `commonAncestorContainer` property pointed to an ancestor of
    the text node

In this scenario, the `wholeTextNodesInRange` function would split the
text node where the range started and add the piece on the right-hand
side of the split to the list of text nodes to wrap in highlight
elements. The `forEachNodeInRange` iterator would then visit the
right-hand text node a _second_ time because it is the _next node_ it
its iteration order. This resulted in the text node being added twice to
the `textNodes` list returned by `wholeTextNodesInRange`.

To resolve this, rewrite the loop in `wholeTextNodesInRange` so that it
visits each text node in the range only once, even if the node is split
mid-iteration. In doing so a small optimization was made by setting the
`NodeIterator`'s filter to `NodeFilter.SHOW_TEXT` in order to skip any
non text nodes.