add types for async functions

src/functions.d.ts

@@ -1,6 +1,10 @@
 
 export type Func<T extends any = any> = (...args: any[]) => T;
 
+export type AsyncFunc<T extends any = any> = (...args: any[]) => PromiseLike<T>;
+
+export type AsyncResult<T extends AsyncFunc> = Awaited<ReturnType<T>>;
+
 export type Params<T extends Func> = T extends (...args: infer P) => any ? P : never;
 
 export type FirstParam<T extends Func> = T extends (first: infer F, ...args: any[]) => any ? F : never;