In `package.json` engine is set to `>=9.0.0`.

Added a missing comma and used an actual macro so that the MathJax example runs right out of the box upon a copy/paste.

When using the totalTime-based pacing calculation, a presenter may
inadvertently set totalTime and per-slide data-timing attributes in
such a way that the pacing time for some slides is impossibly low or
even negative.

Add a check to ensure that the pacing on a slide never falls below a
configurable minimum, defaulting to 0. Display an alert if the
pacing for any slide(s) falls below the threshold.

new API methods for getting horizontal/vertical slides, force linear navigation for one-dimensional decks