feat(async): support signal on deadline() like delay()

[lambdalisue]

As the subject suggests, I have added support for the signal parameter in the deadline() function. This change is useful to clean up dangling promises for tests like:

Deno.test("...", async () => {
  const controller = new AbortController();
  const { signal } = controller;
  assertEquals(await Promise.race([
    deadline(new Promise(() => {}), 100, { signal }),
    Promise.resolve("Hello"),
  ], "Hello");
  controller.abort();
});

Without the signal parameter, the above test fails with the following error:

error: Leaking async ops:
  - 1 async operation to sleep for a duration was started in this test, but never completed. This is often caus
ed by not cancelling a `setTimeout` or `setInterval` call.
To get more details where ops were leaked, run again with --trace-ops flag.

Note that the comment on deadline suggests using AbortSignal.timeout(...) instead. However, the signal itself cannot be manually aborted (yet), so deadline() is still useful in such cases.

[kt3k]

LGTM. Thanks!