Add modules support (#1015)

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

src/ftxui/screen/string.cppm

@@ -0,0 +1,22 @@
+/**
+ * @file string.cppm
+ * @brief Module file for string functions of the Screen module
+ */
+
+module;
+
+#include <ftxui/screen/string.hpp>
+
+export module ftxui.screen.string;
+
+/**
+ * @namespace ftxui
+ * @brief The FTXUI ftxui:: namespace
+ */
+export namespace ftxui {
+    using ftxui::to_string;
+    using ftxui::to_wstring;
+    using ftxui::string_width;
+    using ftxui::Utf8ToGlyphs;
+    using ftxui::CellToGlyphIndex;
+}