common: use native MultiByteToWideChar

[angt]

std::codecvt_utf8<wchar_t> is deprecated and produces warnings:

common/common.cpp:792:31: warning: 'codecvt_utf8<wchar_t>' is deprecated [-Wdeprecated-declarations]
  792 |     std::wstring_convert<std::codecvt_utf8<wchar_t>> converter;
      |

Please, review carefully as I’ve used Gemini to generate utf8_to_wstring() and I'm absolutely not a Windows dev.

[angt]

found this, so I think we are good:

llama.cpp/ggml/src/ggml.c

Lines 538 to 556 in 2e1c9cd

	#ifdef _WIN32
	static wchar_t * ggml_mbstowcs(const char * mbs) {
	int wlen = MultiByteToWideChar(CP_UTF8, 0, mbs, -1, NULL, 0);
	if (!wlen) {
	errno = EINVAL;
	return NULL;
	}
	wchar_t * wbuf = GGML_MALLOC(wlen * sizeof(wchar_t));
	wlen = MultiByteToWideChar(CP_UTF8, 0, mbs, -1, wbuf, wlen);
	if (!wlen) {
	GGML_FREE(wbuf);
	errno = EINVAL;
	return NULL;
	}
	return wbuf;
	}
	#endif