Add __main__ module with await-compatible console

[wbadart]

I recently learned that python -m asyncio drops you into an
interpreter session that knows how to deal with await expressions. For
example,

$ python -m asyncio
asyncio REPL 3.10.6 [...] on darwin
Use "await" directly instead of "asyncio.run()".
Type "help", "copyright", "credits" or "license" for more information.
>>> import asyncio
>>> await asyncio.sleep(1); print("hi")  # prints after one second
hi

This quickly became an important part of my async debugging toolbox.

I've also been learning Trio recently, but I've been missing my async
console.

This PR takes the script from asyncio.__main__ and adapts it to
use a Trio run loop. With this patch applied, users can run python -m trio to drop into an interactive interpreter session that lets them
directly await Trio-based async expressions.

$ python -m trio
Trio 0.21.0+dev, Python 3.10.6 [...] on darwin
Use "await" directly instead of "trio.run()".
Type "help", "copyright", "credits" or "license" for more information.
>>> import trio
>>> await trio.sleep(1); print("hi")  # prints after one second
hi

I'm very new to Trio, so I'm sure my approach is suboptimal. In
particular, I'd really appreciate feedback from the team/ community on:

• not using threading with a brand new trio.run to evaluate
  awaitable expressions. I found that if I simply used
  self.nursery.start_soon in the same thread as runcode, await
  expressions wouldn't get the chance to be evaluated until the session
  was shutting down (might have been blocked by reading keyboard input,
  if I had to guess). Could there be a solution in trio's threading
  tools?
• what documentation updates should accompany this PR
• what sort of tests I should add (since this change implements a brand
  new code path that won't be touched by any existing or future usage of
  trio itself, I wasn't sure if it would even be appropriate to try
  and test it)
• anything else to align this addition to the Trio way!

Many thanks for your consideration.

[codecov]

Codecov Report

Attention: 48 lines in your changes are missing coverage. Please review.

Comparison is base (78c55aa) 99.64% compared to head (a43a9f3) 99.37%.

Additional details and impacted files

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  Lines       17634    17682      +48     
  Branches     3172     3180       +8     
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  Hits        17572    17572              
- Misses         43       91      +48     
  Partials       19       19              

Files	Coverage Δ
src/trio/__main__.py	0.00% <0.00%> (ø)