gh-116738: Make _codecs module thread-safe

[swtaarrs]

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 two mutexes to codecs_state, one to synchronize operations on search_path, and one to synchronize initialization.

• _PyCodecRegistry_Init() is now _PyCodecRegistry_EnsureInit(), and is called unconditionally to ensure initialization, rather than only when initialization is needed. When it returns 0 (as opposed to -1 to indicate an error), the PyObject * members of codecs_state can be read without further synchronization.

• Operations that mutate codecs.search_path must be performed while holding codecs.search_path_mutex. This allows PyCodec_Unregister() to search for and delete a specific item from the list. 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.

• Issue: Audit all built-in modules for thread safety #116738

[colesbury]

This looks pretty good. Two comments below

[colesbury]

This looks good. A few comments below.

[swtaarrs]

This should be ready to go as of my last commit - let me know if any other changes are needed.