Fix inputhook implementation to be compatible with asyncio.run().

`asyncio.get_event_loop()` got deprecated. So we can't install an event loop
with inputhook upfront and then use in in prompt_toolkit. Instead, we can take
the inputhook as an argument in `Application.run()` and `PromptSession.run()`,
and install it in the event loop that we create ourselves using
`asyncio.run()`.