Added worldmap road forking

[Semphriss]

Levels with multiple endings will now only open their assigned path.

This has been applied only to the Icy Island worldmap (in the main game).

As of this commit:

• Each of Fork in the Road's endings will enable the player to continue their worldmap journey only through their corresponding path.
  • That is, if you finish the level with the top exit, you may only continue through the upwards road; if you finish the level with the bottom exit, you may only continue through the downwards road.
• Icy Valley's main exit will only enable the main road. Its secret exit will enable both the main road and the secret area, as well as make the secret pathway visible using the new semi-visible pathways.

This method uses the state Squirrel object as well as worlmap logic similar to the underground handler to accomplish this.