Add icon for R files (#8223)

Add an icon from https://www.svgrepo.com/svg/340612/logo-r-script for .r

<img width="1392" alt="307467014-63f68791-9d74-4bd1-8065-3698665f7c15"
src="https://github.com/zed-industries/zed/assets/93369643/ce24615c-6946-479a-8660-663bf83a7dde">

Credits For Image: @moshyfawn

Release Notes:

- Added R logo