Investigate using line2byte for highlighting

[isovector]

Allegedly line2byte in vim does the line offsetting that we'd like.

[isovector]

Also, the byteidx function seems to do the right translation given a string. Furthermore, byte2line is almost exactly the buffer lookup we want.

[isovector]

I think the technique here is to use byte2line to get the line, call line2byte on that, subtract the two to get the line offset, and then use byteidx on that to get the actual BufferOffset.