Use foldcase instead of lowercase for case-insensitive P6Regex

When comparing text case-insensitively the correct way is to fold case,
instead of just lowercasing the string. Otherwise things like ligatures
and other characters who may have special upper and lowercase forms will
not be put in an easily comparable format.

Author: samcv

src/QRegex/P6Regex/Actions.nqp

@@ -735,11 +735,11 @@ class QRegex::P6Regex::Actions is HLL::Actions {
                 }
                 elsif $RXim {
                     my $c := nqp::chr(nqp::ordbaseat(~$_[0], 0));
-                    $str := $str ~ nqp::lc($c) ~ nqp::uc($c);
+                    $str := $str ~ nqp::fc($c) ~ nqp::uc($c);
                 }
                 elsif $RXi {
                     my $c := ~$_[0];
-                    $str := $str ~ nqp::lc($c) ~ nqp::uc($c);
+                    $str := $str ~ nqp::fc($c) ~ nqp::uc($c);
                 }
                 elsif $RXm {
                     $str := $str ~ nqp::chr(nqp::ordbaseat(~$_[0], 0));