Simpervisor overrides the existing signal handlers

### Proposed change
Simpervisor defines its own [atexit](https://github.com/jupyterhub/simpervisor/blob/main/simpervisor/atexitasync.py) implementation to register signal handlers. The issue is that sets its own signal handler for `SIGINT` and `SIGTERM` "potentially" overriding any existing handlers.

An example use case when we use [jupyter-server-proxy](https://github.com/jupyterhub/jupyter-server-proxy) to start managed applications. It uses simpervisor behind the hood to supervise the proxy application. By the time simpervisor registers signal handlers, there are already existing handlers for [sigint](https://github.com/jupyter-server/jupyter_server/blob/32df893ef2fdd40a102e73a361c4280dbf042b71/jupyter_server/serverapp.py#L2185-L2193) and [sigterm](https://github.com/jupyter-server/jupyter_server/blob/32df893ef2fdd40a102e73a361c4280dbf042b71/jupyter_server/serverapp.py#L2239-L2242) by `ServerApp`. So, simpervisor overrides these handlers registered by `ServerApp`. This can be easily verified by running a JupyterLab instance with `jupyter-server-proxy` installed. Start a simple web server from JupyterLab using `jupyter-server-proxy` and then doing a Ctrl+C on the terminal that is running JupyterLab will shutdown the Lab immediately without the usual prompt of shutdown [confirmation](https://github.com/jupyter-server/jupyter_server/blob/32df893ef2fdd40a102e73a361c4280dbf042b71/jupyter_server/serverapp.py#L2199-L2237).

The signal handlers of the `ServerApp` cleans up the [kernels](https://github.com/jupyter-server/jupyter_server/blob/32df893ef2fdd40a102e73a361c4280dbf042b71/jupyter_server/serverapp.py#L2558-L2571) and [extensions](https://github.com/jupyter-server/jupyter_server/blob/32df893ef2fdd40a102e73a361c4280dbf042b71/jupyter_server/serverapp.py#L2573-L2582) before terminating itself. So, we lose this cleaning up tasks due to simpervisor overriding the handlers. We noticed on our JupyterHub deployment on our HPC platform that this can have a undesirable side effects of proxy processes lingering around even after `ServerApp` is killed.

### Who would use this feature?
Anyone who is using `simpervisor` along with Jupyter `ServerApp`.

### (Optional): Suggest a solution
Ensure that we register the existing signal handlers in [_handlers](https://github.com/jupyterhub/simpervisor/blob/ffd8787fd440995b8dc542959d36f76fd05e65c0/simpervisor/atexitasync.py#L9) list, if they exist.

Start in simpervisor/atexitasync.py, especially the _handlers list and the signal registration path. Reproduce the issue with JupyterLab and jupyter-server-proxy by starting a proxied web server and pressing Ctrl+C. Done means existing SIGINT and SIGTERM handlers remain effective so ServerApp shutdown confirmation and cleanup still occur.

Written by the indexing model from the issue text.