[C API] Move undocumented private _PyArg C API to pycore_modsupport.h internal C API

If a 3rd party C extension uses one of these functions, we should consider adding a clean, documented and tested public function to replace it.

Private functions and structures:

• _PyArg_BadArgument()
• _PyArg_CheckPositional()
• _PyArg_NoKeywords()
• _PyArg_NoPositional()
• _PyArg_ParseStack()
• _PyArg_ParseStackAndKeywords()
• _PyArg_Parser structure
• _PyArg_UnpackKeywords()
• _PyArg_UnpackKeywordsWithVararg()
• _PyArg_UnpackStack()
• _Py_ANY_VARARGS()

Linked PRs

• gh-110964: Remove private _PyArg functions #110966
• gh-110964: clinic: refactor output_templates() #110982
• gh-110964: clinic: pass clinic argument to bad_argument() #110984