[Fiber] render boundary in fallback if it contains a new stylesheet during sync update (#28965)

Updates Suspensey instances and resources to preload even during urgent
updates and to potentially suspend.

The current implementation is unchanged for transitions but for sync
updates if there is a suspense boundary above the resource/instance it
will be rendered in fallback mode instead.

Note: This behavior is not what we want for images once we make them
suspense enabled. We will need to have forked behavior here to
distinguish between stylesheets which should never commit when not
loaded and images which should commit after a small delay

Author: gnoff

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

@@ -3154,7 +3154,6 @@ export function mayResourceSuspendCommit(resource: Resource): boolean {
 }
 
 export function preloadInstance(type: Type, props: Props): boolean {
-  // Return true to indicate it's already loaded
   return true;
 }
 
@@ -3163,10 +3162,11 @@ export function preloadResource(resource: Resource): boolean {
     resource.type === 'stylesheet' &&
     (resource.state.loading & Settled) === NotLoaded
   ) {
-    // we have not finished loading the underlying stylesheet yet.
+    // Return false to indicate this resource should suspend
     return false;
   }
-  // Return true to indicate it's already loaded
+
+  // Return true to indicate this resource should not suspend
   return true;
 }