Update gitattributes to force LF

[MichaelChirico]

Follow-up to #6378 and #6369

My understanding is this works as follows:

1. Every file is treated as a plain text file, with LF line endings (i.e. \n)
2. Except for certain binary file types: .ico, .mo, .png, .RData
3. And except in the inst/tests directory, where files are binary (-text) by default
4. Except for .Rraw files in inst/tests, which should also be treated as plain text LF

[Anirban166]

LGTM in terms of the syntax! (can just keep adding to the list of binaries to be excluded from being treated as text files)