Add support for nxxm.

[nxxm](https://nxxm.github.io)

src/ftxui/screen/string.cpp

@@ -0,0 +1,14 @@
+#include "ftxui/screen/string.hpp"
+
+#include <codecvt>
+#include <locale>
+
+std::string to_string(const std::wstring& s) {
+  std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> converter;
+  return converter.to_bytes(s);
+}
+
+std::wstring to_wstring(const std::string& s) {
+  std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> converter;
+  return converter.from_bytes(s);
+}