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.