enforce lf line ending for shim

[zkamvar]

This should avoid the situation we found on @klbarnes20's machine and confirmed on @froggleston's machine where Git is forcing CRLF line endings. Instead of having both set core.autocrlf=input, this .gitattributes file should enforce this file having lf line endings.

@klbarnes20, can you test this on your computer by cloning this branch:

git clone --branch lf-line-endings-shim https://github.com/carpentries/sandpaper

and then running this in R (inside the just-cloned repo)

stopifnot(tools::md5sum("inst/pkgdown/shim.R") == "230853fec984d1a0e5766d3da79f1cea")

[zkamvar]

I have confirmed with @klbarnes20 that this works!

[klbarnes20]

Thanks @zkamvar!