Add a global flag for disable view transitions

[lauri865]

It would be great to have a global flag or a method to disable view transitions.

Use cases could be to disable view transitions on low battery or as a user perference on lower grade hardware . Achieveable in userland technically, but since it's a flag in many individual hooks, it becomes easily cumbersome to implement.

[lauri865]

While it is achievable with CSS, it seems that invoking startViewTransition adds a visible delay to routing regardless. Don't know if it's react-router's end, or browser's implementation