Compute line:col visual width using 1-based counting
Compute line:col visual width using 1-based counting

Fixes #1618

The error store is based on (0, 0) counting. But only in the panel, we present
the errors using natural (1, 1) line/column counting.

Here the width was computed wrong, e.g. for `start=9` we computed a `1`, but
the user is actually seeing a `10` which is 2 chars.

SublimeLinter/SublimeLinter3,SublimeLinter/SublimeLinter3