gh-107603: Argument Clinic: Only include pycore_gc.h if needed (#108726)

Argument Clinic now only includes pycore_gc.h if PyGC_Head is needed,
and only includes pycore_runtime.h if _Py_ID() is needed.

* Add 'condition' optional argument to Clinic.add_include().
* deprecate_keyword_use() includes pycore_runtime.h when using
  the _PyID() function.
* Fix rendering of includes: comments start at the column 35.
* Mark PC/clinic/_wmimodule.cpp.h and
  "Objects/stringlib/clinic/*.h.h" header files as generated in
  .gitattributes.

Effects:

* 42 header files generated by AC no longer include the internal C
  API, instead of 4 header files before. For example,
  Modules/clinic/_abc.c.h no longer includes the internal C API.
* Fix _testclinic_depr.c.h: it now always includes pycore_runtime.h
  to get _Py_ID().