Leverage new API functions added in v1.7.5

[cyber5]

We can make the code more robust by replacing old solutions with the following functions:

• Room:IsMirrorWorld()
• Room:HasCurseMist()
• Level:IsAscent()

While we're at it, RoomType.ROOM_SECRET_EXIT appears to have been fixed finally.

[cwilliams5]

Also supposedly they fixed custom sounds not working. This opens the possibility of solving the jingle interrupting music bug by treating jingles as custom sound effects that play over the music.

However I see Taz has stopped working on Isaac Mods (no judgement, dont blame him, I'm half way there), so unless someone forks this and carries the torch its going to break more over time as game is updated.

[cyber5]

I have a fork that I've used to submit pull requests. I can work on maintaining this mod for the foreseeable future, as time allows.