Implement GDScript debug_parse_stack_level_expression, allow to
1、Assign 'this' to current node
2、Can print node's member value by:
-> p this.member
-> p this["rect/pos"]
-> p this.get_pos()
3、Can change node's member value by:
-> e this.member = 1
-> e this["rect/pos"] = Vector2(2, 2)