Fix parsing of multi-line extended regex

[wesQ3]

In Perl 5.26+, the regex quotelike operator accepts a second x modifier to allow whitespace inside of character classes. This can improve readability of complex character classes.

Currently TM Perl syntax does not parse extended regex with /xx correctly when broken across multiple lines.

# single line is ok
my $single = $foo =~ s/[ \N{NO-BREAK SPACE} \r \n {} " ]/ /grxx;
# multi-line highlights ok without second x
# this is syntactically invalid but useful for comparison
my $multi  = $foo =~ s/[
        \N{NO-BREAK SPACE}
        \r \n
        {}
        "
        ]/ /grx;
# multi line extended breaks when second x is added
my $multi_xx  = $foo =~ s/[
        \N{NO-BREAK SPACE}
        \r \n
        {}
        "
        ]/ /grxx;
# rest of file is now broken because of unmatched second x

A simple adjustment to the extended regex 'end' condition to allow the second x fixes the parsing error.