toggle method
      
void
toggle()
      
     
    
Toggles the current boolean state.
Changes true to false and false to true, then notifies all listeners
of the change. This is equivalent to setting value = !value but provides
a more semantic API for toggle operations.
Example:
final controller = ToggleController(false);
controller.toggle(); // value is now true
controller.toggle(); // value is now false
Implementation
void toggle() {
  value = !value;
}