Define a LineWriter type for use in OutputWriter

[sirosen]

closes #1696

Hat tip to @graingert for the original! I'm just carrying it over the finish line.

Because this is primarily a readability edit of existing logic, I have not included a changelog or new tests. (Open to changing upon request/feedback.)

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To abstract away the distinction between writing lines in dry-run mode
and non-dry-run mode, define a small protocol for a "line writer" which
takes and emits lines of text as strs.

This idea originates in #1696 , but was not completed at the time. On
review, the change is perfectly sound and can be implemented with some
slightly clearer types using a protocol to define the interface common to
the two codepaths.
The abstraction improves readability, especially by moving a finally
block into a context manager __exit__ function.

Contributor checklist

• Included tests for the changes.
• A change note is created in changelog.d/ (see changelog.d/README.md
  for instructions) or the PR text says "no changelog needed".

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• If no changelog is needed, apply the bot:chronographer:skip label.
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  (following Semantic Versioning).