Move scroll handling to GDScript

Just expose the underlying libtsm methods from gdnative, but write all
the logic in gdscript. Makes it much easier to customize and update.

addons/godot_xterm/native/src/terminal.h

@@ -56,6 +56,9 @@ public:
 
   void write(String data);
 
+  void sb_up(int num);
+  void sb_down(int num);
+
   enum UpdateMode {
     DISABLED,
     AUTO,