gh-104469: Convert _testcapi/watchers.c to use Argument Clinic (#104503)

Remove boilerplate code by converting the following functions:

- _testcapi.watch_dict
- _testcapi.unwatch_dict
- _testcapi.watch_type
- _testcapi.unwatch_type
- _testcapi.set_func_defaults_via_capi
- _testcapi.set_func_kwdefaults_via_capi