Add support for nxxm.

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

include/ftxui/dom/requirement.hpp

@@ -0,0 +1,26 @@
+#ifndef FTXUI_DOM_REQUIREMENT_HPP
+#define FTXUI_DOM_REQUIREMENT_HPP
+
+#include "ftxui/screen/box.hpp"
+
+namespace ftxui {
+
+struct Requirement {
+  // The required size to fully draw the element.
+  struct { int x = 0; int y = 0; } min;
+
+  // How much flexibility is given to the component.
+  struct { int x = 0; int y = 0; } flex;
+
+  // Frame.
+  enum Selection {
+    NORMAL = 0,
+    SELECTED = 1,
+    FOCUSED = 2,
+  } selection = NORMAL;
+  Box selected_box;
+};
+
+}; // namespace ftxui
+
+#endif /* end of include guard: FTXUI_REQUIREMENT_HPP */