gh-139353: Rename formatter_unicode.c to unicode_formatter.c (#139723)

* Move Python/formatter_unicode.c to Objects/unicode_formatter.c.
* Move Objects/stringlib/localeutil.h content into
  unicode_formatter.c. Remove localeutil.h.
* Move _PyUnicode_InsertThousandsGrouping() to unicode_formatter.c
  and mark the function as static.
* Rename unicode_fill() to _PyUnicode_Fill() and export it in
  pycore_unicodeobject.h.
* Move MAX_UNICODE to pycore_unicodeobject.h as _Py_MAX_UNICODE.

Python/fileutils.c

@@ -2,6 +2,7 @@
 #include "pycore_fileutils.h"     // fileutils definitions
 #include "pycore_runtime.h"       // _PyRuntime
 #include "pycore_pystate.h"       // _Py_AssertHoldsTstate()
+#include "pycore_unicodeobject.h" // _Py_MAX_UNICODE
 #include "osdefs.h"               // SEP
 
 #include <stdlib.h>               // mbstowcs()
@@ -50,9 +51,6 @@ extern int winerror_to_errno(int);
 int _Py_open_cloexec_works = -1;
 #endif
 
-// The value must be the same in unicodeobject.c.
-#define MAX_UNICODE 0x10ffff
-
 // mbstowcs() and mbrtowc() errors
 static const size_t DECODE_ERROR = ((size_t)-1);
 #ifdef HAVE_MBRTOWC
@@ -123,7 +121,7 @@ is_valid_wide_char(wchar_t ch)
 {
 #ifdef HAVE_NON_UNICODE_WCHAR_T_REPRESENTATION
     /* Oracle Solaris doesn't use Unicode code points as wchar_t encoding
-       for non-Unicode locales, which makes values higher than MAX_UNICODE
+       for non-Unicode locales, which makes values higher than _Py_MAX_UNICODE
        possibly valid. */
     return 1;
 #endif
@@ -132,7 +130,7 @@ is_valid_wide_char(wchar_t ch)
         return 0;
     }
 #if SIZEOF_WCHAR_T > 2
-    if (ch > MAX_UNICODE) {
+    if (ch > _Py_MAX_UNICODE) {
         // bpo-35883: Reject characters outside [U+0000; U+10ffff] range.
         // The glibc mbstowcs() UTF-8 decoder does not respect the RFC 3629,
         // it creates characters outside the [U+0000; U+10ffff] range: