feat(ui): emit "msg_clear" event after clearing the screen (#34035)

Problem:  ext_messages cannot tell when the screen was cleared, which is
          needed to clear visible messages. An empty message is also
          never emitted, but clears messages from the message grid.
Solution: Repurpose the "msg_clear" event to be emitted when the screen
          was cleared. Emit an empty message with the `empty` kind to
          hint to a UI to clear the cmdline area.

src/nvim/drawscreen.c

@@ -264,6 +264,9 @@ void screenclear(void)
     msg_grid_invalid = false;
     clear_cmdline = true;
   }
+  if (ui_has(kUIMessages)) {
+    ui_call_msg_clear();
+  }
 }
 
 /// Unlike cmdline "one_key" prompts, the message part of the prompt is not stored