Fixed some problems with show all errors

- If the column was 0 and the line was indented, the arrow was placed in the wrong place.
- It now reliably selects the correct region.
Column fixes/enhancements...

- <col> in regex may now be a string, which is then converted to its length. This provides for the common case where a linter shows the position on the offending line with a caret. In such cases, <col> can be set to capture the characters before the caret.
- In some cases, linters report a position which is beyond the end of the line. So the column is now pinned to the line length.