[Contact Form] Add cascading fade in on scroll

[kjellr]

Summary

Add cascading fade in on scroll animation for the Contact Form Section. based on explorations made in #2141. Pair with UX to define the ideal strong outcome.

Details

• Reveal content in cascade for a more interesting flow, adapt if we need to reduce the noise. First intention would be to have each column reveal individually. Could be informed by the Image with text section and be applied on the Product information section on the Product template
• 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.