chore(surveys): expose current question index on survey-box (#3353)

* feat(surveys): expose current question index on survey-box

Adds a data-question-index attribute to the .survey-box wrapper so
consumers rendering surveys via the API can reliably tell which
question is currently displayed. Works for every question type,
including link questions which render no input or rating element.

* test(surveys): cover data-question-index update on next question