Using the `--IPKernelApp.trio_loop=True` doesn't appear to set the main loop to use `trio`

To reproduce:

```
% ipython --IPKernelApp.trio_loop=True
Python 3.11.6 | packaged by conda-forge | (main, Oct 3 2023, 10:37:07) [Clang 15.0.7 ]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.24.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import trio

In [2]: await trio.sleep(1)
...
RuntimeError: must be called from async context
```

However the following does work, which is not expected:

```
% ipython --IPKernelApp.trio_loop=True
Python 3.11.6 | packaged by conda-forge | (main, Oct 3 2023, 10:37:07) [Clang 15.0.7 ]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.24.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import asyncio

In [2]: await asyncio.sleep(1)

In [3]:
```

I could have missed something obvious though...

Reproduce the command with --IPKernelApp.trio_loop=True and compare the trio and asyncio await cases. Trace how ipykernel handles the trio_loop setting and identify why trio.sleep is outside an async context; done means trio awaits work under the flag and the observed asyncio behavior matches the intended semantics.

Written by the indexing model from the issue text.