Add support for nxxm.

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

src/ftxui/dom/separator.cpp

@@ -0,0 +1,56 @@
+#include "ftxui/dom/node.hpp"
+
+namespace ftxui {
+
+using ftxui::Screen;
+
+class Separator : public Node {
+ public:
+  Separator() {}
+  ~Separator() override {}
+  void ComputeRequirement() override {
+    requirement_.min.x = 1;
+    requirement_.min.y = 1;
+  }
+
+  void Render(Screen& screen) override {
+    bool is_column = (box_.x_max == box_.x_min);
+    bool is_line = (box_.y_min == box_.y_max);
+
+    wchar_t c = U'+';
+    if (is_line && !is_column)
+      c = U'─';
+    else
+      c = U'│';
+
+    Pixel p;
+    p.character = c;
+    RenderWithPixel(screen, p);
+  }
+
+  void RenderWithPixel(Screen& screen, Pixel pixel) {
+    for (int y = box_.y_min; y <= box_.y_max; ++y) {
+      for (int x = box_.x_min; x <= box_.x_max; ++x) {
+        screen.PixelAt(x, y) = pixel;
+      }
+    }
+  }
+};
+
+class SeparatorWithPixel : public Separator {
+ public:
+  SeparatorWithPixel(Pixel p) : p(p) {}
+  ~SeparatorWithPixel() override {}
+  void Render(Screen& screen) override { RenderWithPixel(screen, p); }
+  Pixel p;
+};
+
+std::unique_ptr<Node> separator() {
+  return std::make_unique<Separator>();
+}
+
+std::unique_ptr<Node> separator(Pixel pixel) {
+  return std::make_unique<SeparatorWithPixel>(pixel);
+}
+
+};  // namespace ftxui