Implement async Event Loop w/ipykernel==7.0.0 alpha

[so-rose]

We have a lot of issues and bugs with dangling sockets, etc. right now, when restarting the kernel in Blender.

Luckily, upstream seems to have fixed this somewhat: The new version of ipykernel, which currently has an alpha, will be implemented using an async event loop.

Our task is then to implement a clean async event loop for Blender (using a modal operator; it's been done before), then use this event loop to more cleanly allow the new ipykernel to run without the rather nasty hacks we use currently. Notably, a stop() method is now available.

When ipykernel==7.0.0 actually releases, it should of course be migrated to.