Better Unicode Support for Position Updates

[j-mie6]

Parsley 4 introduced some improved Unicode support for caret width; however, this support is reasonably limited. Ideally, position information would also be adjusted with respect to the number of codepoints traversed and not characters. This is potentially expensive, so should be handled in a light-weight way in relevant places: like the caret width, this may not be 100% accurate in all cases.