Non-native line breaks in `dfetch init` and `dfetch freeze` manifests

[spoorcc]

As mentioned by @sach-edna

When dfetch dumps a manifest (https://github.com/dfetch-org/dfetch/blob/main/dfetch/manifest/manifest.py#L231) for instance during dfetch init then the line_break argument is not specified. This seems to default to LF on all platforms.
It is possible to provide a explicit line_break according to https://pyyaml.org/wiki/PyYAMLDocumentation#reference.

We could use os.linesep to make the manifest native on each platform.

This would impact dfetch init and dfetch freeze