Add modules support (#1015)

Add experimental C++20 module suppport.
Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

src/ftxui/dom/linear_gradient.cppm

@@ -0,0 +1,18 @@
+/**
+ * @file linear_gradient.cppm
+ * @brief Module file for the LinearGradient struct of the Dom module
+ */
+
+module;
+
+#include <ftxui/dom/linear_gradient.hpp>
+
+export module ftxui.dom.linear_gradient;
+
+/**
+ * @namespace ftxui
+ * @brief The FTXUI ftxui:: namespace
+ */
+export namespace ftxui {
+    using ftxui::LinearGradient;
+}