Aside from overall simplifying `CharToColumnMapper` this fixes 2 bugs:
* The backward search loop may have iterated 1 column too far,
because it didn't stop at `*current <= *target`, but rather at
`*(current - 1) <= *target`. This issue was only apparent when
surrogate pairs were being used in a row.
* When the target offset is that of a trailing surrogate pair
the forward search loop may have iterated 1 column too far.
It's somewhat unlikely for this to happen since this code is
only used through ICU, but you never know.
This is a continuation of PR #16775.
