chore: fix not being able to discard changes to .png files

[mikavilpas]

After cloning on osx, a file in this repository is constantly reported as having been modified.

Demo:

https://github.com/tree-sitter-grammars/tree-sitter-markdown/assets/300791/ffba7783-ad1a-453e-bcdd-b8cd04400216

ChatGPT thinks this is caused by .gitattributes specifying all files to be text files with lf line endings:

[MDeiml]

Thanks!