Add interfaces from CSS Animations Level 2 and CSS Transitions Level 2

[birtles]

These specs define the CSSAnimation and CSSTransition interfaces respectively which are implemented in all three major browser engines.

I believe we simply need to add the following URLs to idlSources.json:

• https://drafts.csswg.org/css-animations-2/
• https://drafts.csswg.org/css-transitions-2/

[birtles]

We shouldn't add animationComposition and animationTimeline from CSS Animations Level 2, however. No-one implements that.

[saschanaz]

Added by #1034

@github-actions close

[github-actions]

Closing because @saschanaz is one of the code-owners of this repository.