Csf-plugin: Move source to docs.source.originalSource

[shilman]

Issue: N/A
Telescopes on #20603

What I did

CSF-plugin extracts the original story source from the CSF file and writes it into the parameters for the story for use in the source block.

Originally, it wrote to the storySource.source parameter for historical reasons. This used to be provided by source-loader, which was shared between addon-docs and addon-storysource.

Now, csf-plugin makes addon-docs completely independent from addon-storysource, so we can refactor the Source block in #20603 to use a different parameter.

This PR writes to docs.source.originalSource. #20603 will also need to be updated accordingly.

How to test

• See attached unit tests

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[tmeasday]

Lgtm