Add support for canceling transitions

[Edvinas01]

Currently the package doesn't support loading a collection while another collection is loading. While this makes sense as canceling a scene mid-load introduces a lot of edge cases, it is a bit annoying to use with long transitions.

For example, when having a long fade-out transition, the player might want to open the pause menu and exit the game (load menu scene). However, the long transition will prevent that as the ScriptableSceneController.IsLoading will be set to true.

Make it so that transitions would not contribute/could be canceled and another collection could be loaded on top.