Add feature for animation `iterationComposite` (web animations)

[ddbeck]

Specification

https://drafts.csswg.org/web-animations-2/#dom-keyframeeffect-iterationcomposite

Description

I learned of this via #3494, so I don't know a lot about this feature.

The description would probably just lift from MDN a bit, something like:

The iterationComposite property of a KeyframeEffect object sets how an animation's property value changes, accumulating or overriding, on each step of the animation.

Documentation

https://developer.mozilla.org/en-US/docs/Web/API/KeyframeEffect/iterationComposite

Browser support

My best guess is that this would be a feature for the BCD key api.KeyframeEffect.iterationComposite.

Other stuff:

• Firefox 80 release notes for developers - Mozilla | MDN
• 1652676 - Ship animation compositing to release channels
• Web Animations Level 2: iterationComposite - Chrome Platform Status
• Intent to Prototype: Web Animations Level 2: iterationComposite

[ddbeck]

I'm not quite sure if this is an actual feature, or just a later addition to the web animations feature. I'm also not sure how this relates to api.Element.animate.options_iterationComposite_parameter, if at all.