Don't change newline on Windows

[nhz2]

This PR ensures the same bytes are written out on all systems. This is especially useful on Windows when using WSL, where files written in Windows must be read in Linux.

I also added a .gitattributes file to prevent GitHub Windows CI from changing the file endings: https://code.visualstudio.com/docs/devcontainers/tips-and-tricks#_resolving-git-line-ending-issues-in-containers-resulting-in-many-modified-files

[tkonolige]

I'm having trouble finding any documentation on if matrix market files use \r\n on windows. The best I've found is scipy's implementation here: https://github.com/scipy/scipy/blob/v1.14.0/scipy/io/_fast_matrix_market/fast_matrix_market/include/fast_matrix_market/fast_matrix_market.hpp#L33. They just use \n everywhere, so I'm fine following their example.