Add getComputedStyle helper to ViewTransition refs

This is also sometimes useful to read the style of the pseudo-element itself
without an animation.

Author: sebmarkbage

packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js

@@ -2079,6 +2079,7 @@ export function stopGestureTransition(transition: RunningGestureTransition) {
 interface ViewTransitionPseudoElementType extends Animatable {
   _scope: HTMLElement;
   _selector: string;
+  getComputedStyle(): CSSStyleDeclaration;
 }
 
 function ViewTransitionPseudoElement(
@@ -2132,6 +2133,14 @@ ViewTransitionPseudoElement.prototype.getAnimations = function (
   }
   return result;
 };
+// $FlowFixMe[prop-missing]
+ViewTransitionPseudoElement.prototype.getComputedStyle = function (
+  this: ViewTransitionPseudoElementType,
+): CSSStyleDeclaration {
+  const scope = this._scope;
+  const selector = this._selector;
+  return getComputedStyle(scope, selector);
+};
 
 export function createViewTransitionInstance(
   name: string,