py/dynruntime: Add mp_obj_exception_init function to define C exception.

[dpgeorge]

This allows defining custom exceptions in C in dynamic native modules. An example of this is added to the existing features4 example.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.39%. Comparing base (a919ce2) to head (d532f96).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #14497   +/-   ##
=======================================
  Coverage   98.39%   98.39%           
=======================================
  Files         161      161           
  Lines       21204    21204           
=======================================
  Hits        20864    20864           
  Misses        340      340           

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[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:   +24 +0.003% standard
      stm32:    +4 +0.001% PYBV10
     mimxrt:    +8 +0.002% TEENSY40
        rp2:    +8 +0.002% RPI_PICO
       samd:    +4 +0.002% ADAFRUIT_ITSYBITSY_M4_EXPRESS