PR# 17608 Pretty printer rewrites manifest strings spanning multiple lines

The pretty printer rewrites manifest strings that span multiple lines.
It make two changes - it removes the leading and trailing % on each line,
concatenating all the lines into one longer string, and it replaces at least
some characters with % escape sequences (only one I noticed so far was
a Tab character replaced by %T).

I think this is incorrect behavior.  We have some SQL queries that we wrote
(before the advent of verbatim strings) with continued lines, such as:

   query: STRING = "select some_value %N%
                    % from my_table %N%
                    % where design_no = :1"

Some of the queries are quite long and would be unreadable if joined into one
single long string.  Of course, we could replace them with verbatim strings
but the pretty printer should handle continued strings better.

Run ec -pretty on attached class.

Attachments for problem report #17608