Given a function from suggestion item to string and a search string, the highlight letters function should return an array of Tuple2s. Each tuple marks the start index and length of a range that should be bold in the suggestion string that is printed (using the same toString method) in the list.
The default version of this function returns exactly one range to highlight,
which starts at indexOf the search string in the stringified suggestion item
and has a length equal to the length of the search string.