Change line-ending default to auto

[MichaReiser]

Summary

This PR changes the format.line-ending default from lf to auto.

The motivation for defaulting to lf is to ensure all files in a project use the same line ending, regardless of the file author's platform. However, it has the downside that it results in unexpected line-ending changes for existing projects that use a mixture of \n and \r\n and projects that use windows only.
We haven't received much feedback on the lf default, but there has been at least one occurrence where a user was confused about why Ruff changed all lines.
They struggled to understand the changes because many diff viewers hide whitespace changes by default.
We also put them in an uncomfortable position because they were unfamiliar with line endings (the difference between \n and \r\n) and didn't know what option to choose. So they defaulted to auto because it seems like the most clever option that hopefully does the right thing for them.

Breaking Change

While this is a breaking change because it changes the configuration, it doesn't require a new minor version because the formatter is still in alpha (excluded from versioning).

Test Plan

I added a new integration test that ensures the formatter preserves the line ending choice when run without any options.

[MichaReiser]

Current dependencies on/for this PR:

• main
  • PR Change line-ending default to auto #8057 👈

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PR Check Results

Ecosystem

✅ ecosystem check detected no changes.

[charliermarsh]

👍 👍

[Cjkjvfnby]

Thank you!

I got the problem with this default on my regular project on Windows. Git manages line endings for me: https://docs.github.com/en/get-started/getting-started-with-git/configuring-git-to-handle-line-endings. So changes are committed in UNIX style and code on my machine is in Windows style.

[Cjkjvfnby]

Although auto is a great option. For my projects, I'd prefer to use native. It ensures that all my files are done the same way. All projects I have seen with mixed line endings we more resulted in the absence of the tools for fixing it, it was not something that was required.