stacked here-docs not handled

[ossilator]

as documented in https://perldoc.perl.org/perlop#EOF, it is possible to have multiple adjacent here-documents:

   print <<"foo", <<"bar"; # you can stack them
I said foo.
foo
I said bar.
bar

this isn't highlighted properly - the second here-doc isn't treated as such, which of course leads to all kinds of "interesting" followup effects when it contains quotes, etc.

go to https://github.com/git/git/blob/v2.39.2/git-send-email.perl#L835 for a real-life demo.