bytes_to_str() Win32: explicitly list UTF-32* as unsupported

I think it is better to admit that they're not implemented yet than to
pretend they don't exist. Explicitly returning `KAITAI_CP_UNSUPPORTED`
serves like a "TODO" comment, while omitting them pretends everything is
fine. They are also in KSC's
[`EncodingList`](https://github.com/kaitai-io/kaitai_struct_compiler/blob/5832a81a48e10c3c207748486e09bd58b9aa4000/shared/src/main/scala/io/kaitai/struct/EncodingList.scala#L21-L22),
so the fact that the `KS_STR_ENCODING_WIN32API` mode doesn't support
them is a limitation and we should be able to see it directly in the
code.

kaitai/kaitaistream.cpp

@@ -880,6 +880,16 @@ int kaitai::kstream::encoding_to_win_codepage(const char *src_enc) {
         return KAITAI_CP_UTF16BE;
     if (enc == "UTF-16LE")
         return KAITAI_CP_UTF16LE;
+    if (enc == "UTF-32BE") {
+        // It has a code page number 12001 assigned to it, but it's "available only to
+        // managed applications", so we can't use it.
+        return KAITAI_CP_UNSUPPORTED;
+    }
+    if (enc == "UTF-32LE") {
+        // It has a code page number 12000 assigned to it, but it's "available only to
+        // managed applications", so we can't use it.
+        return KAITAI_CP_UNSUPPORTED;
+    }
     if (enc == "ISO-8859-1")
         return 28591;
     if (enc == "ISO-8859-2")