detect existing line endings when writing baseline.json

It can be frustrating when you have a git configuration to use different line endings to you system default, and mypy write baseline with system line endings instead of git ones. This causes vscode to have an error popup saying that the line endings are incorrect. For this change we will attempt to detect the existing line endings in the baseline file and use that when re-writing.

Start by locating the code that rewrites baseline.json and inspect how it currently chooses line endings. Reproduce the mismatch with a repository using non-system Git line endings, then verify that rewriting baseline.json preserves the existing file's line-ending style.

Written by the indexing model from the issue text.