Add support for nxxm.

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

src/ftxui/dom/vbox.cpp

@@ -0,0 +1,72 @@
+#include "ftxui/dom/node.hpp"
+#include "ftxui/dom/elements.hpp"
+
+namespace ftxui {
+
+class VBox : public Node {
+ public:
+  VBox(Elements children) : Node(std::move(children)) {}
+  ~VBox() {}
+
+  void ComputeRequirement() {
+    requirement_.min.x = 0;
+    requirement_.min.y = 0;
+    requirement_.flex.x = 0;
+    requirement_.flex.y = 1;
+    for (auto& child : children) {
+      child->ComputeRequirement();
+      if (requirement_.selection < child->requirement().selection) {
+        requirement_.selection = child->requirement().selection;
+        requirement_.selected_box = child->requirement().selected_box;
+        requirement_.selected_box.y_min += requirement_.min.y;
+        requirement_.selected_box.y_max += requirement_.min.y;
+      }
+      requirement_.min.y += child->requirement().min.y;
+      requirement_.min.x =
+          std::max(requirement_.min.x, child->requirement().min.x);
+    }
+  }
+
+  void SetBox(Box box) {
+    Node::SetBox(box);
+
+    int flex_sum = 0;
+    for (auto& child : children)
+      flex_sum += child->requirement().flex.y;
+
+    int space = box.y_max - box.y_min + 1;
+    int extra_space = space - requirement_.min.y;
+
+    int remaining_flex = flex_sum;
+    int remaining_extra_space = extra_space;
+
+    int y = box.y_min;
+    for (auto& child : children) {
+      if (y > box.y_max)
+        break;
+
+      Box child_box = box;
+      child_box.y_min = y;
+
+      child_box.y_max = y + child->requirement().min.y - 1;
+
+      if (child->requirement().flex.y && remaining_extra_space > 0) {
+        int added_space = remaining_extra_space * child->requirement().flex.y /
+                          remaining_flex;
+        remaining_extra_space -= added_space;
+        remaining_flex -= child->requirement().flex.y;
+        child_box.y_max += added_space;
+      }
+      child_box.y_max = std::min(child_box.y_max, box.y_max);
+
+      child->SetBox(child_box);
+      y = child_box.y_max + 1;
+    }
+  }
+};
+
+std::unique_ptr<Node> vbox(Elements children) {
+  return std::make_unique<VBox>(std::move(children));
+}
+
+};  // namespace ftxui