Add support for nxxm.

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

include/ftxui/screen/string.hpp

@@ -0,0 +1,9 @@
+#include <string>
+
+std::string to_string(const std::wstring& s);
+std::wstring to_wstring(const std::string& s);
+
+template<typename T>
+std::wstring to_wstring(T s) {
+  return to_wstring(std::to_string(s));
+}