fix(types): add loadAsync to Loader interface

[Methuselah96]

When trying to use @types/three@0.149.0 with R3F, I get these type errors:

node_modules/@react-three/fiber/dist/declarations/src/core/hooks.d.ts:12:108 - error TS2344: Type 'InstanceType<L>["loadAsync"]' does not satisfy the constraint '(...args: any) => any'.

12 export declare type LoaderReturnType<T, L extends LoaderProto<T>> = T extends unknown ? Awaited<ReturnType<InstanceType<L>['loadAsync']>> : T;
                                                                                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

node_modules/@react-three/fiber/dist/declarations/src/core/hooks.d.ts:12:108 - error TS2536: Type '"loadAsync"' cannot be used to index type 'InstanceType<L>'.

12 export declare type LoaderReturnType<T, L extends LoaderProto<T>> = T extends unknown ? Awaited<ReturnType<InstanceType<L>['loadAsync']>> : T;
                                                                                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is caused by the loadAsync method being removed from THREE.Loader in three-types/three-ts-types#311, since there's not a good consistent type definition for it.

This fixes the situation by adding the loadAsync method here.

This change change should be backwards-compatible, I tested it with both @types/three@0.149.0 and @types/three@0.139.0.

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