Add object separator when using YAML output

[SteveL-MSFT]

PR Summary

When using YAML output, add --- separator between objects

• rename write_output() to write_object() which more accurately describes the function
• add parameter to write_object() indicating whether to emit YAML object separator

PR Context

Fix #628

[Copilot]

Copilot reviewed 1 out of 2 changed files in this pull request and generated 1 comment.

Files not reviewed (1)

• dsc/tests/dsc_args.tests.ps1: Language not supported

[SteveL-MSFT]

Need to wait for #634 which fixes the unit test failures unrelated to this change