LibVT: Allow updating the window progress via an escape sequence

You can now request an update of the terminal's window progress by
sending this escape sequence:

<esc>]9;<value>;<max_value>;<escape><backslash>

I'm sure we can find many interesting uses for this! :^)

Kernel/TTY/VirtualConsole.cpp

@@ -307,6 +307,11 @@ void VirtualConsole::set_window_title(const StringView&)
     // Do nothing.
 }
 
+void VirtualConsole::set_window_progress(int, int)
+{
+    // Do nothing.
+}
+
 void VirtualConsole::terminal_did_resize(u16 columns, u16 rows)
 {
     ASSERT(columns == 80);