[view-transitions-2] Add method to get all animations of a ViewTransition object

[mozilla-apprentice]

Resolutions were made for csswg-drafts/#9908.

[view-transitions-2] Add method to get all animations of a ViewTransition object

• RESOLVED: Add a pseudoElement option to the element.getAnimations() dict arg. subtree:true/false acts on that pseudo as the root
• RESOLVED: Add .transitionRoot readonly property to VT object, reflectring the element hosting the VT.

Discussion.

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[mozilla-apprentice]

https://bugzilla.mozilla.org/show_bug.cgi?id=1935557

[BorisChiou]

We need a bug for web-animations change, i.e. add pseudoElement in getAnimations(), though this is for web-animations level 1.