Default .gitattributes for crystal init

[HertzDevil]

#13479 adds a .gitattributes to this repository. It would be helpful if new projects also do the same.

The main thing to add is:

*.cr text eol=lf

It ensures that checkins convert CRLF to LF (although there shouldn't be any as long as one runs the formatter) and checkouts preserve line endings, even without having to set core.autocrlf. If for some reason we want to preserve line endings on checkins as well, it would be *.cr -text instead.

That's about it because the lib directory is ignored; shard repositories do not source in dependencies like Crystal itself does.