Allow control over view transition animation duration

[felixarntz]

The View Transitions plugin should provide control over the transition animation duration. See also https://wordpress.org/support/topic/requested-features-6/

Unlike other animation settings, the duration is something that likely applies for any animation. So we may want to implement as a globally supported argument, differently from the existing arguments per animation. That said, it may still be necessary for each animation to implement support individually. This can be decided as part of the pull request, as it's a close-to-code decision. Conceptionally at least, the idea of a consistent argument across any kind of animation makes sense for this.