examples/usercmodule: Include API headers in the C++ example.

[stinos]

Make the CI builds compile the public API as C++ to catch accidental introductions of incompatible code.

See #14323

[projectgus]

Adding CI build coverage for C++ is good, but adding it as code in a user-facing example (that's primarily something someone can take, learn from, and adapt into their own code) seems not ideal. It's a lot of extra noise for someone who's just starting out with the C API.

MicroPython doesn't (to my knowledge) have a category of "build tests" (i.e. things that are functionally examples, but exist for project development and test purposes.) Maybe this could be the use case for the first one?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.39%. Comparing base (9c7f065) to head (a65076a).
Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #14370   +/-   ##
=======================================
  Coverage   98.39%   98.39%           
=======================================
  Files         161      161           
  Lines       21204    21204           
=======================================
  Hits        20864    20864           
  Misses        340      340           

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[stinos]

Fair point. Practically: create a tests/ subdirectory for this code then? Preference for a name?

i.e. things that are functionally examples, but exist for project development and test purposes

The tests/ directory has a bunch of things which fall under that category imo. The socket multi tests etc.