Feature: LinearGradient color decorator. (#592)

Based on the existing color decorators, create new ones to apply a gradient effect on the DOM.

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

examples/dom/linear_gradient.cpp

@@ -0,0 +1,26 @@
+#include <ftxui/dom/elements.hpp>  // for bgcolor, operator|, operator|=, text, center, Element
+#include <ftxui/dom/linear_gradient.hpp>  // for LinearGradient::Stop, LinearGradient
+#include <ftxui/screen/screen.hpp>        // for Full, Screen
+#include <memory>                         // for allocator, shared_ptr
+
+#include "ftxui/dom/node.hpp"  // for Render
+#include "ftxui/screen/color.hpp"  // for Color, Color::DeepPink1, Color::DeepSkyBlue1, Color::Yellow, ftxui
+
+int main(int argc, const char* argv[]) {
+  using namespace ftxui;
+  auto document = text("gradient") | center;
+
+  document |= bgcolor(LinearGradient()
+                          .Angle(45)
+                          .Stop(Color::DeepPink1)
+                          .Stop(Color::DeepSkyBlue1));
+  auto screen = Screen::Create(Dimension::Full(), Dimension::Full());
+  Render(screen, document);
+  screen.Print();
+
+  return 0;
+}
+
+// Copyright 2023 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.