Add loadAsync to Loader interface

The `loadAsync` method on `THREE.Loader` was removed in three-types/three-ts-types#311, since there's no consistent type definition for it.

This fixes the situation by adding the `loadAsync` method here.

packages/fiber/src/core/hooks.tsx

@@ -14,6 +14,7 @@ export interface Loader<T> extends THREE.Loader {
     onProgress?: (event: ProgressEvent) => void,
     onError?: (event: ErrorEvent) => void,
   ): unknown
+  loadAsync(url: string, onProgress?: (event: ProgressEvent) => void): Promise<T>
 }
 
 export type LoaderProto<T> = new (...args: any) => Loader<T extends unknown ? any : T>