feat(term): add select method

Adds select() method to Terminal. Method behaves the same way as
TextEdit's select method.

addons/godot_xterm/native/src/terminal.h

@@ -74,6 +74,8 @@ namespace godot
 
     void clear();
 
+    void select(const int p_from_line, const int p_from_column, const int p_to_line, const int p_to_column);
+
     String copy_all();
     String copy_selection();
     void set_copy_on_selection(const bool p_enable);