Replace highlight_lines with highlight_range with slightly improved result

[dhardy]

This is a partial fix for https://github.com/kas-gui/kas/issues/232.

[dhardy]

Really, the highlight_line method wants improving too. Ideas:

• Possibly sorting runs into visual order
• Attempting to merge runs where they are visually contiguous (non-trivial for tabs and justified alignment where the run blocks don't physically touch)
• Possibly for this purpose RunPart should contain more data