Add support for nxxm.

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

src/ftxui/dom/flex.cpp

@@ -0,0 +1,58 @@
+#include "ftxui/dom/node.hpp"
+#include "ftxui/dom/elements.hpp"
+
+namespace ftxui {
+
+class Flex : public Node {
+ public:
+  Flex() {}
+  Flex(Element child) : Node(unpack(std::move(child))) {}
+  ~Flex() override {}
+  void ComputeRequirement() override {
+    requirement_.min.x = 0;
+    requirement_.min.y = 0;
+    if (!children.empty()) {
+      children[0]->ComputeRequirement();
+      requirement_ = children[0]->requirement();
+    }
+    requirement_.flex.x = 1;
+    requirement_.flex.y = 1;
+  }
+
+  void SetBox(Box box) override {
+    if (children.empty())
+      return;
+    children[0]->SetBox(box);
+  }
+};
+
+class NotFlex : public Flex {
+ public:
+  NotFlex() {}
+  NotFlex(Element child) : Flex(std::move(child)) {}
+  ~NotFlex() override {}
+  void ComputeRequirement() override {
+    requirement_.min.x = 0;
+    requirement_.min.y = 0;
+    if (!children.empty()) {
+      children[0]->ComputeRequirement();
+      requirement_ = children[0]->requirement();
+    }
+    requirement_.flex.x = 0;
+    requirement_.flex.y = 0;
+  }
+};
+
+std::unique_ptr<Node> filler() {
+  return std::make_unique<Flex>();
+}
+
+std::unique_ptr<Node> flex(Element child) {
+  return std::make_unique<Flex>(std::move(child));
+}
+
+std::unique_ptr<Node> notflex(Element child) {
+  return std::make_unique<NotFlex>(std::move(child));
+}
+
+};  // namespace ftxui