[web-animations-1] Add parameter for fetching all animations on a subtree

[birtles]

This fixes #2058.

[annevk]

So the idea is to not tackle shadow roots at all? Will you add tests to ensure nothing from shadow roots is returned?

[birtles]

So the idea is to not tackle shadow roots at all? Will you add tests to ensure nothing from shadow roots is returned?

Right. Yes, I need to update the implementation in Firefox to match the spec terms and I plan to add WPT at that point. I'll make sure to add one for shadow roots at the same time.