Feature resizable spilt with custom separator (#583)

* Feature: ResizableSplit with custom separator

This resolves:
    https://github.com/ArthurSonzogni/FTXUI/issues/580

Co-authored-by: Pin Loon Lee <pinloon_0428@hotmail.com>

include/ftxui/dom/direction.hpp

@@ -0,0 +1,17 @@
+#ifndef FTXUI_DOM_DIRECTION_HPP
+#define FTXUI_DOM_DIRECTION_HPP
+
+namespace ftxui {
+enum class Direction {
+  Up = 0,
+  Down = 1,
+  Left = 2,
+  Right = 3,
+};
+
+}  // namespace ftxui
+
+#endif  // FTXUI_DOM_DIRECTION_HPP
+// 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.