Add clear and sb_reset

Clearing the terminal removes all lines in the scrollback buffer except
for the most recent.

With sb_reset, the terminal will return the scrollback buffer to the
bottom when the user starts typing if they have previously scrolled up.

addons/godot_xterm/native/src/terminal.h

@@ -57,6 +57,8 @@ public:
 
   void sb_up(int num);
   void sb_down(int num);
+  void sb_reset();
+  void clear_sb();
 
   void start_selection(Vector2 position);
   void select_to_pointer(Vector2 position);