WPT details: Tests in wpt_internal. Will migrate out to external/wpt as the feature ships
This feature allows View transition API use to be customized for different types of transitions. Specifically, this adds an ability to add "types" to `startViewTransition` call which will identify the types of the transition. As well, it will match a pseudo-class, called `:active-view-transitions(...)` with a parameter matching the type for the duration of the view transition. Combined these two features provide a way for the author to declare several view transitions once and only trigger one at a time. See example usage in the spec: https://drafts.csswg.org/css-view-transitions-2/#the-active-view-transition-pseudo