Allow clients to query if the terminal emulator support styled underlines

kitty/modes.h

@@ -76,5 +76,8 @@
 #define BRACKETED_PASTE_START "\033[200~"
 #define BRACKETED_PASTE_END  "\033[201~"
 
+// Styled underlines
+#define STYLED_UNDERLINES (2016 << 5)
+
 // Extended keyboard protocol
 #define EXTENDED_KEYBOARD (2017 << 5)

kitty/screen.c

@@ -923,6 +923,8 @@ report_mode_status(Screen *self, unsigned int which, bool private) {
         KNOWN_MODE(EXTENDED_KEYBOARD);
         KNOWN_MODE(FOCUS_TRACKING);
 #undef KNOWN_MODE
+        case STYLED_UNDERLINES:
+            ans = 3; break;
     }
     if (snprintf(buf, sizeof(buf) - 1, "\x1b[%s%u;%u$y", (private ? "?" : ""), which, ans)) callback("write_to_child", self, buf, 0);
 }