Add response_index to survey-multi-choice data

[stephenmjerge]

Summary

• add response_index array to survey-multi-choice trial data (unanswered = -1)
• expose option indices via data attributes
• update simulation to use indices when clicking
• add tests for duplicate/empty options and unanswered questions
• document response_index

Testing

• npx jest -c packages/plugin-survey-multi-choice/jest.config.cjs

Fixes #3639

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🦋 Changeset detected

Latest commit: 7c1813a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@jspsych/plugin-survey-multi-choice	Patch

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