proposal: heredoc comments to allow `+/` in comments, eg from urls or documented unittests

[Timothee Cour]

same exact idea as motivation for delimited strings
(https://dlang.org/spec/lex.html#delimited_strings)

```

auto heredoc = q"EOS
This is a multi-line
heredoc string
EOS"
;

/"EOC
This is a multi-line
heredoc comment allowing
/+ documented unittests containing nesting comments +/
and weird urls like https://gcc.gnu.org/onlinedocs/libstdc++/faq.html
EOS"/

```