gh-116738: Make _codecs module thread-safe (#117530)

The module itself is a thin wrapper around calls to functions in
`Python/codecs.c`, so that's where the meaningful changes happened:

- Move codecs-related state that lives on `PyInterpreterState` to a
  struct declared in `pycore_codecs.h`.

- In free-threaded builds, add a mutex to `codecs_state` to synchronize
  operations on `search_path`. Because `search_path_mutex` is used as a
  normal mutex and not a critical section, we must be extremely careful
  with operations called while holding it.

- The codec registry is explicitly initialized as part of
  `_PyUnicode_InitEncodings` to simplify thread-safety.

Python/pystate.c

@@ -843,9 +843,7 @@ interpreter_clear(PyInterpreterState *interp, PyThreadState *tstate)
     }
 
     PyConfig_Clear(&interp->config);
-    Py_CLEAR(interp->codec_search_path);
-    Py_CLEAR(interp->codec_search_cache);
-    Py_CLEAR(interp->codec_error_registry);
+    _PyCodec_Fini(interp);
 
     assert(interp->imports.modules == NULL);
     assert(interp->imports.modules_by_index == NULL);