Wrong line endings used when building YAML edits

Trying to track down some failing tests on Windows in the analysis server, and it seems to be caused by this:

https://github.com/dart-lang/yaml_edit/blob/2a9a11bee120d507d61d501c34585440be8c12b6/lib/src/strings.dart#L277

It's using `\n` but never gets rewritten to `lineEnding`, so the resulting YAML has a mix of `\n` and `\r\n` on Windows.