Make ecpg parse.pl more robust with braces

Make ecpg parse.pl more robust with braces

When parse.pl processes braces, it does not take into account that
braces could also be their own token if single quoted ('{', '}').
This is not currently used but a future patch wants to make use of it.

This fixes that by using lookaround assertions to detect the quotes.

To make sure all Perl versions in play support this and to avoid
surprises later on, let's give this a spin on the buildfarm now.  It
can exist independently of future work.

Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/a855795d-e697-4fa5-8698-d20122126567@eisentraut.org