Enable end-of-line normalization with Git attributes

[staticdev]

This setting converts line endings to UNIX-style (LF), whenever text files are checked into and out of the repository.

Without this setting, the pre-commit hook for Prettier fails on the Windows runner. GitHub Actions configures git with core.autocrlf = true, because this is the default setting in Git for Windows.

https://prettier.io/docs/en/options.html#end-of-line
actions/checkout#135