Add support for italics (#1009)

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

include/ftxui/screen/pixel.hpp

@@ -17,6 +17,7 @@ struct Pixel {
       : blink(false),
         bold(false),
         dim(false),
+        italic(false),
         inverted(false),
         underlined(false),
         underlined_double(false),
@@ -27,6 +28,7 @@ struct Pixel {
   bool blink : 1;
   bool bold : 1;
   bool dim : 1;
+  bool italic : 1;
   bool inverted : 1;
   bool underlined : 1;
   bool underlined_double : 1;