Fix tabulation columns in diagnostic output

[brendanzab]

Currently codespan-reporting treats tabs as being a constant width, but this is incorrect, as @legendiguess notes:

The other would be to only adjust the underline calculation to account for tabs in line, but I'm not sure if display of "\tt" is always 5 ascii characters.

\t move next characters to the next tabulation column. so basically it's adds from 1 to 4 ascii characters, dependent on caret position off last tabulation column

We'll need to change the source writer and the width calculuation to properly support tabulation columns.