Use lowercase filenames.

src/ftxui/screen/box.cppm

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