`ignore-nested-functions` should work with coroutines

### Describe the feature you'd like

I'd like `ignore-nested-functions` to work with coroutines (i.e. `async def`). However if either function is actually a coroutine, it does not work

### Is your feature request related to a problem?

I'm frustrated when I have to add docstrings to unimportant functions and I can't avoid it just because of how they are defined i.e. there is nothing materially different between a function and coroutine when it comes to docstrings and their necessity (or lack thereof)

### Your Environment

- `interrogate` version(s) (`interrogate --version`: 1.5.0
- Operating System(s): MacOS Big Sur
- Python version(s): 3.9.7