Add support for italics (#1009)

Co-authored-by: Ken Reneris <ms/devops kreneris@microsoft.com>
Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

include/ftxui/dom/elements.hpp

@@ -95,6 +95,7 @@ Element canvas(std::function<void(Canvas&)>);
 // -- Decorator ---
 Element bold(Element);
 Element dim(Element);
+Element italic(Element);
 Element inverted(Element);
 Element underlined(Element);
 Element underlinedDouble(Element);