Add forwards fill mode to animations in view transition fixture (facebook#34502)

It turns out that View Transitions can sometimes overshoot and then we
need to ensure it fills. It can otherwise sometimes flash in Chrome.

This is something users might hit as well.

Author: sebmarkbage

fixtures/view-transition/src/components/Transitions.module.css

@@ -56,10 +56,10 @@
 }
 
 ::view-transition-new(.enter-slide-right):only-child {
-  animation: enter-slide-right ease-in 0.25s;
+  animation: enter-slide-right ease-in 0.25s forwards;
 }
 ::view-transition-old(.exit-slide-left):only-child {
-  animation: exit-slide-left ease-in 0.25s;
+  animation: exit-slide-left ease-in 0.25s forwards;
 }
 
 :root:active-view-transition-type(navigation-back) {