Add (?^...) regex construct

Add (?^...) regex construct

This adds (?^...) to signify to use the default regex modifiers for the
cluster or embedded pattern-match modifier change.  The major purpose of
this is to simplify regex stringification, so that "^" is output in
place of "-xism".  As a result, the stringification will not change in
the future when new regex modifiers are added, so tests, etc. that rely
on a particular stringification will have to change now, but never
again.

Code that needs to work properly with both old- and new-style regexes
can use something like the following:

    # Accept both old and new-style stringification
    my $modifiers = (qr/foobar/ =~ /\Q(?^/) ? '^' : '-xism';

This construct is Ben Morrow's idea.