Add `\r\n` to string_continue grammar

[ModProg]

Not sure if this is necessary, but I wasn't 100% sure that crlf line endings would be supported (though in hindsight would be strange if they weren't)

[ehuss]

Thanks for the PR! I think I would prefer to add a holistic solution for CRLF normalization instead of trying to address individual places. There are various points noted in #626 about CRLF normalization (and bare CR handling) that should probably be added as a whole (I believe per https://github.com/rust-lang/rust/issues/62865 that it is translated globally, but I haven't investigated in a long while).

[ModProg]

makes sense, it was just that having only the \n here sounded incomplete. But as there is probably no case where CRLF will be preserved in source code there is a better solution than listing it.

[ehuss]

Closing as superseded by #1459. Thanks!