fix: forgot to export duration fn

Changelog: fix

src/index.ts

@@ -3,5 +3,6 @@ import { delay, sleep } from "./delay";
 import { timeout } from "./timeout";
 import { deferred } from "./deferred";
 import { retry } from "./retry";
+import { duration } from "./duration";
 
-export { to, delay, sleep, timeout, deferred, retry };
+export { to, delay, sleep, timeout, deferred, retry, duration };

test/build/duration.spec.ts

@@ -0,0 +1,8 @@
+import { describe, expect, it } from "vitest";
+import { duration } from '../../dist/index.js'
+
+describe("duration", () => {
+  it("must be a function", () => {
+    expect(typeof duration).toBe("function");
+  });
+});