Improved Inline Highlighting

[mitsuhiko]

Currently inline highlighting is always using word (or unicode word if available) level diffing. There are some expectations that these are character level diffs (https://github.com/mitsuhiko/similar-asserts/issues/1).

There are advantages and disadvantages to both. In the current form I'm not happy with how character diffs look like since similar does not have semantic cleanups like dissimilar has. Though even in dissimilar I found that character level diffs can be awkward when put on some diffs. This is a meta item to figure out what a good strategy could look like here.