[ENG-3476]Setting State Vars that are not defined should raise an error

[ElijahAhianyo]

import reflex as rx

class State(rx.State):
   value: int = 0
   def change_value(self):
       self.value = 10 # valid
       self.num = 5   # error
       self._num = 10 # error
       self.__num = 10 # valid

[linear]

ENG-3476 Setting State Vars that are not defined should raise an error