feat: pin rocks to prevent updates

[mrcjkb]

Part 1 of #272.

A :Rocks [pin|unpin] <plugin> command might follow up in a later PR.

[github-actions[bot]]

Review Checklist

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• For example, fix(installer): some installer bugfix

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If applicable:

• [x] Tested
  • [x] Tests have been added.
  • [x] Tested manually (steps in PR description).
• [x] Updated documentation.