examples/usercmodule: Add a sub-package example.

This demonstrates how to add a sub-package in a user c module, as well
as how to define the necessary qstrs and enable the feature in the build.

This is used by the unix coverage build to test this feature.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

examples/usercmodule/cppexample/examplemodule.c

@@ -4,7 +4,7 @@
 // See example.cpp for the definition.
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(cppfunc_obj, cppfunc);
 
-// Define all properties of the module.
+// Define all attributes of the module.
 // Table entries are key/value pairs of the attribute name (a string)
 // and the MicroPython object reference.
 // All identifiers and strings are written as MP_QSTR_xxx and will be