Intent to prototype and ship: animation property of AnimationEvent and TransitionEvent

[Kanaru Sato]

Summary:
Add animation property to AnimationEvent and TransitionEvent. This prop has the corresponding CSSAnimation and CSSTransition respectively. This is useful for identifying the animation tied to the event. 

Bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1929118

Specification:
AnimationEvent: https://drafts.csswg.org/css-animations-2/#interface-animationevent-idl

TransitionEvent: https://drafts.csswg.org/css-transitions-2/#interface-transitionevent-idl

Standards Body:
CSS WG

Platform Coverage:
All

Preference:
N/A

DevTools Bug:
N/A

Extensions Bug:
N/A

Use Counter:
N/A

Standards-Positions Discussion:
https://github.com/w3c/csswg-drafts/issues/9010

Other Browsers:
    Blink: not yet
    WebKit: not yet

web-platform-tests:
https://phabricator.services.mozilla.com/D293582