- Expand capabilities to handle both async and sync iterables
- There is a problem in the original functionality around the `stop-on-error` test that needs to be resolved
  - The test has a bug in that neither p-map nor the test mapper function, which is provided for that specific test and is not the common version, await the calling of the delay() function for item 2
  - This means that item 2 will only ever return a Promise, never a value
  - When this is fixed to await the function call, it causes the returned results to change from `[1, 3]` to `[1, 3, 2]`
  - Fixing this test makes it clear that the `concurrency setup` loop does not wait for mappers, so the only way to make this test fail would be to have enough items that eventually the initial iteration could not reach the last items in the test array before the 100 ms delay on the exception expires
  - With `concurrency = 1` this test behaves as expected since there are not unlimited unawaited promises created
  - Let me know your thoughts on how to resolve this as it becomes more apparent with the behavior changes needed for asyncIterable (where the concurrency setup iteration of asyncIterable has to be awaited to prevent inifinte runners from being created)
  - The test, in the state it's in in this PR, does not actually demonstrate that stop on error works as intended

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

- Also distinguish from items returning a promise
- Update the TS types to indicate that iterated items can actually be a `Promise`