ToAsyncEnumerable should return source if source implements IAsyncEnumerable

#### Bug

If `source` implements `IAsyncEnumerable`, `ToAsyncEnumerable` should just cast and return it.

> Which subcomponent library (Ix, Async.Ix)?

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/ToAsyncEnumerable.cs

> Which library version?

6.0.1

> What are the platform(s), environment(s) and related component version(s)?

Windows

> What is the use case or problem?

Calling `ToAsyncEnumerable` on an `IQueryable` that implements `IAsyncEnumerable`.

> What is the expected outcome?

Returns `source as IAsyncEnumerable`.

> What is the actual outcome?

Forces non-async behaviour for all interaction with `source` - exactly the opposite of the desired behaviour.

> What is the stacktrace of the exception(s) if any?

N/A.

> Do you have a code snippet or project that reproduces the problem?

```
IAsyncEnumerable source = ...
IEnumerable enumerable = (IEnumerable)source;
var target = enumerable.ToAsyncEnumerable();

// target should be the same object as source
```