[Blog posts] and [Collection list]: Add cascading fade in on scroll animation

[melissaperreault]

Summary

Add cascading fade in on scroll animation for Blog Posts and Collection List sections based on explorations made in #2141. Pair with UX to define the ideal strong outcome.

Details

For both

• Reveal content in cascade for a more interesting flow, adapt if we need to reduce the noise. Should follow all cards related sections. See decisions made in animate Featured collection, Related products and Product grid sections #2372
• The setting will be applied from the new Animations theme setting category, below Layout and above Buttons. The new setting label is offered via a checkbox and its label is; Reveal sections on scroll.
• The setting will be off by default for the initial release, than turned on for the next one.