LibVT+Kernel: Add support for setting cursor styles

This commit introduces support for 3 new escape sequences:
1. Stop blinking cursor mode
2. `DECTCEM` mode (enable/disable cursor)
3. `DECSCUSR` (set cursor style)

`TerminalWidget` now supports the following cursor types: block,
underline and vertical bar. Each of these can blink or be steady.
`VirtualConsole` ignores these (just as we were doing before).

Kernel/TTY/VirtualConsole.cpp

@@ -387,6 +387,11 @@ void VirtualConsole::emit(const u8* data, size_t size)
         TTY::emit(data[i], true);
 }
 
+void VirtualConsole::set_cursor_style(VT::CursorStyle)
+{
+    // Do nothing
+}
+
 String VirtualConsole::device_name() const
 {
     return String::formatted("tty{}", minor());