✨ Add `storyPreviousPageIndex` variable

[newmuis]

Fixes #35539.

Documentation to be added separately (when feature is generally available).

[amp-owners-bot[bot]]

Hey @gmajoulet! These files were changed:

extensions/amp-story/1.0/variable-service.js

[danielstockton]

Any idea when this might be included in a release?

[gmajoulet]

@newmuis should we create a new PR or are you able to rebase a PR created from the web UI?

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.