Perl 6 shebang should be recognized

Upstream nano has this in `perl.nanorc`:
```
header "^#!.*perl[-0-9._]*"
```

I think that's what is making it recognize most perl 6 scripts as perl5.

perl6.nanorc should recognize perl6 shebang, and FWIW perl5 nanorc should be tweaked if this is submitted upstream.