[ENHANCEMENT/Modding] Allow custom exit callbacks in OptionsState and StoryMenuState

[Starexify]

Description

This PR allows modification of the exiting state in OptionsState and StoryMenuState.
Allowing mods to modify the behavior when exiting them instead of being hardcoded to MainMenuState and having to use workarounds that might not always work properly and feel choppy because it goes into the MainMenu for a bit before switching the state again.

Example Script

import funkin.modding.module.ScriptedModule;
import funkin.modding.events.ScriptEvent;
import funkin.ui.options.OptionsState;
import flixel.FlxG;
import funkin.ui.title.TitleState;
import funkin.ui.story.StoryMenuState;

class Testing extends ScriptedModule {
  override public function onStateCreate(event:ScriptEvent) {
    // Set the StoryMenuState exit state to TitleState
    if (!(FlxG.state is StoryMenuState)) return;
    var storymenu:StoryMenuState = FlxG.state;
    storymenu.exitState = () -> new TitleState();

    // Set the OptionsState exit state to TitleState
    if (!(FlxG.state is OptionsState)) return;
    OptionsState.instance.exitState = () -> new TitleState();
  }
}