MA0156 - Use 'Async' suffix when a method returns IAsyncEnumerable\

Sources: MethodsReturningAnAwaitableTypeMustHaveTheAsyncSuffixAnalyzer.cs, MethodsReturningAnAwaitableTypeMustHaveTheAsyncSuffixFixer.cs

Methods that return IAsyncEnumerable<T> should have the Async suffix.

// compliant
IAsyncEnumerable<string> FooAsync() => throw null;

// non-compliant
IAsyncEnumerable<string> Foo() => throw null;