[web-animations-2] Introduce StaggerEffect in GroupEffect

[yi-gu]

In the proposed GroupEffect explainer we introduced StaggerEffect (children effects start with different delays) in addition to the traditional ParallelEffect (all children effects start together) and SequenceEffect (children effects start in turn) to GroupEffect. Notably the StaggerEffect can be achieved by modifying ParallelEffect [1]. However, introducing a separate API may still be a great idea for ergonomic reasons. In addition, most popular libraries (GSock, Angular, AnimateJS, React animated framework etc.) make the stagger effect a separate category.

[1] #4302 (comment)

[birtles]

Discussed with @grorg and @majido and emphasized that it is very common to have a decreasing delay between effects. A constant delay often looks unnatural. It might be possible to apply a timing function to describe the decay on the delay amount (e.g. linearly decreasing, vs exponential fall off).

[birtles]

Furthermore, sometimes these delays are slightly randomized to give a more organic effect where some items appear slightly before those that would otherwise proceed them. I don't expect that could be handled with a timing function, however.