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.

Objects/unicodeobject.c

@@ -15441,7 +15441,11 @@ init_fs_encoding(PyThreadState *tstate)
 PyStatus
 _PyUnicode_InitEncodings(PyThreadState *tstate)
 {
-    PyStatus status = init_fs_encoding(tstate);
+    PyStatus status = _PyCodec_InitRegistry(tstate->interp);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
+    status = init_fs_encoding(tstate);
     if (_PyStatus_EXCEPTION(status)) {
         return status;
     }