gh-139353: Add Objects/unicode_codecs.c file

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• Copy functions from unicodeobject.c:

  • _PyUnicode_UTF8()
  • PyUnicode_UTF8(), PyUnicode_SET_UTF8()
  • PyUnicode_UTF8_LENGTH(), PyUnicode_SET_UTF8_LENGTH()
  • get_latin1_char()
  • findchar()

• Share code with unicodeobject.c:

  • _PyUnicode_FromUCS1()
  • _PyUnicode_FiniEncodings()
  • _PyUnicode_TranslateCharmap()
  • _Py_EncodingMapType;

• Issue: Split large Objects/unicodeobject.c file into smaller files #139353

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Line count:

$ wc -l Objects/unicode_codecs.c Objects/unicodeobject.c 
  6671 Objects/unicode_codecs.c
  8541 Objects/unicodeobject.c
 15212 total

In PR gh-139354, I created 3 files for codecs:

• Objects/unicode_convert.c (835 lines)
• Objects/unicode_codecs_utf.c (2,171 lines)
• Objects/unicode_codecs.c (3,239 lines)