Auto merge of #26362 - mrobinson:animation-animation-events, r=emilio,jdm

Add support for animationend event

This is triggered when an animation finishes. This is a high priority
because it allows us to start rooting nodes with animations in the
script thread.

This doesn't yet cause a lot of tests to pass because they rely on the
existence of `Document.getAnimations()` and the presence of
`animationstart` and `animationiteration` events.

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