Feature: hscroll_indicator (#753)

This is the symetrical of `vscroll_indicator`.

Requested by @ibrahimnasson.

Fixed:https://github.com/ArthurSonzogni/FTXUI/issues/752

src/ftxui/screen/string_test.cpp

@@ -164,4 +164,4 @@ TEST(StringTest, to_wstring) {
   EXPECT_EQ(to_wstring(std::string("🎅🎄")), L"🎅🎄");
 }
 
-}
+}  // namespace ftxui