Add support for nxxm.

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

src/ftxui/dom/size.cpp

@@ -0,0 +1,77 @@
+#include "ftxui/dom/elements.hpp"
+#include "ftxui/dom/node.hpp"
+
+namespace ftxui {
+
+class Size : public Node {
+ public:
+  Size(Element child, Direction direction, Constraint constraint, size_t value)
+      : Node(unpack(std::move(child))),
+        direction_(direction),
+        constraint_(constraint),
+        value_(value) {}
+
+  ~Size() override {}
+
+  void ComputeRequirement() override {
+    Node::ComputeRequirement();
+    requirement_ = children[0]->requirement();
+
+    auto& value = direction_ == WIDTH ? requirement_.min.x : requirement_.min.y;
+
+    switch (constraint_) {
+      case LESS_THAN:
+        value = std::min(value, value_);
+        break;
+      case EQUAL:
+        value = value_;
+        break;
+      case GREATER_THAN:
+        value = std::max(value, value_);
+        break;
+    }
+
+    if (direction_ == WIDTH)
+      requirement_.flex.x = 0;
+    else
+      requirement_.flex.y = 0;
+  }
+
+  void SetBox(Box box) override {
+    Node::SetBox(box);
+
+    if (direction_ == WIDTH) {
+      switch(constraint_) {
+        case LESS_THAN:
+        case EQUAL:
+          box.x_max = std::min(box.x_min + value_ + 1, box.x_max);
+          break;
+        case GREATER_THAN:
+          break;
+      }
+    } else {
+      switch(constraint_) {
+        case LESS_THAN:
+        case EQUAL:
+          box.y_max = std::min(box.y_min + value_ + 1, box.y_max);
+          break;
+        case GREATER_THAN:
+          break;
+      }
+    }
+    children[0]->SetBox(box);
+  }
+
+ private:
+  Direction direction_;
+  Constraint constraint_;
+  int value_;
+};
+
+Decorator size(Direction direction, Constraint constraint, int value) {
+  return [=](Element e) {
+    return std::make_unique<Size>(std::move(e), direction, constraint, value);
+  };
+}
+
+};  // namespace ftxui