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.

Include/internal/pycore_interp.h

@@ -14,6 +14,7 @@ extern "C" {
 #include "pycore_atexit.h"        // struct atexit_state
 #include "pycore_ceval_state.h"   // struct _ceval_state
 #include "pycore_code.h"          // struct callable_cache
+#include "pycore_codecs.h"        // struct codecs_state
 #include "pycore_context.h"       // struct _Py_context_state
 #include "pycore_crossinterp.h"   // struct _xidregistry
 #include "pycore_dict_state.h"    // struct _Py_dict_state
@@ -182,10 +183,7 @@ struct _is {
      possible to facilitate out-of-process observability
      tools. */
 
-    PyObject *codec_search_path;
-    PyObject *codec_search_cache;
-    PyObject *codec_error_registry;
-    int codecs_initialized;
+    struct codecs_state codecs;
 
     PyConfig config;
     unsigned long feature_flags;