Add .gitattributes to fix Windows CI prettier format-check failure

[Copilot]

Fixes the Windows CI failure at https://github.com/coursier/cache-action/actions/runs/25727659691/job/75544511285.

Root cause

On Windows, Git's default behavior is to convert LF line endings to CRLF on checkout. When prettier --check **/*.ts runs, it expects LF (the prettier v3 default), so it fails on the CRLF files.

The workflow already tries to work around this with test "$OSTYPE" != "msys" || npm run format, but this is unreliable: npm scripts on Windows use cmd.exe internally, and the interaction between the bash runner shell and npm's cmd.exe invocation can cause the glob **/*.ts to not reliably match and rewrite the files before the check runs.

Fix

Add a .gitattributes file that forces LF line endings on all platforms:

* text=auto eol=lf

This prevents Git from converting line endings to CRLF on Windows checkout, so the files will always have LF endings and prettier --check will pass consistently.