webassembly: Add native asyncio support

[dpgeorge]

This PR adds a significant portion of the existing MicroPython asyncio module to the webassembly port, using parts of the existing asyncio code and some custom JavaScript parts.

The key difference to the standard asyncio is that this version uses the JavaScript runtime to do the actual scheduling and waiting on events, eg Promise fulfillment, timeouts, fetching URLs.

This implementation does not include asyncio.run(). Instead one just uses asyncio.create_task(..) to start tasks and then returns to the JavaScript. Then JavaScript will run the tasks.

The implementation here tries to reuse as much existing asyncio code as possible, and gets all the semantics correct for things like cancellation and asyncio.wait_for. It's is different to #14193 which reimplemented tasks etc using JavaScript Promise's. That approach is very difficult to get right when trying to implement cancellation (because it's not possible to cancel a JavaScript Promise).

[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.39%. Comparing base (9c7f065) to head (49af8ca).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #14328   +/-   ##
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  Coverage   98.39%   98.39%           
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  Files         161      161           
  Lines       21204    21204           
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  Hits        20864    20864           
  Misses        340      340           

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