Add support for nxxm.

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

tutorial/build-with-nxxm.md

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+# Add dependency
+
+.nxxm/deps
+~~~json
+{
+  "arthursonzogni/ftxui": {}
+}
+~~~
+
+# Example project
+
+main.cpp
+~~~cpp
+#include "ftxui/screen/screen.hpp"
+#include "ftxui/dom/elements.hpp"
+#include <iostream>
+
+int main(int argc, const char *argv[]) {
+  using namespace ftxui;
+  auto document =
+    hbox(
+      text(L"left") | bold | border,
+      text(L"middle") | flex | border,
+      text(L"right") | border
+    );
+  auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
+  Render(screen, document.get());
+
+  std::cout << screen.ToString();
+
+  return 0;
+}
+~~~
+
+# Build
+
+~~~
+nxxm . -t clang-cxx17
+~~~